I've been leveling my new worgen priest as disc/holy since I am most interested in healing and don't like the idea of being a shadow priest. Perhaps I'm weird, but shadow just seems evil; it took me rolling a blood elf to justify making a warlock. I guess I'm ok with overcoming an inherent evil taint in my toons so long as they didn't actively choose it and try to overcome it by doing good. My blood elf warlock was forced to fel magics to survive, my death knight had no say in being resurrected as an abomination, and I suppose worgen to some extent also didn't have a choice in what they were to become or what they did before being "cured". So going shadow just doesn't work for me. That said I decided early on to make my disc spec my "DPS" spec and focus entirely on healing for my holy spec.
Turns out disc does pretty decent damage at lower levels, even at 48 I am still competitive with other DPS in pugs. When I flush out the disc tree and get to take a few points in shadow I'll be able to use spirit for hit and with any luck be able to hold my own in normal dungeons. Apparently I'm also ok with tossing the occasional shadow spell in the heat of battle; chalk it up to the worgen curse, but embracing shadow fully is where I draw the line.
So given that I know I can put out more that my fair share of damage I don't feel the least bit guilty about checking that DPS checkbox when queuing for a random dungeon. I always have healer checked as well, but apparently once in a while the DPS queue is shorter and I get into a dungeon as DPS. So I focus on maximizing damage instead of healing when that happens which pretty much means I use my penance for damage and apply dots instead of tossing a heal to accompany my smite and holy fire spam. In either case atonement does a ton of healing and under normal circumstances covers all the healing. Only during a particularly bad pull is any direct healing required; therein lies the one problem, what does the healer do when I DPS?
One of three things from my experience: roll with it and take a nap, do some DPS in the downtime or throw a fit and drop group. The latter is usually quite hilarious, but I can't help but to feel a teeny bit sorry for them; while I am laughing of course. Honestly there is nothing funnier than a healer just blowing up after a few pulls and typing something profane along the lines of, "screw this, you guys don't even need me", and dropping group. That's like the DPS dropping group because the tank is doing all the damage, which I have seen happen. Although the tank and healer above all the DPS is pretty amusing to look at, but I've never seen a DPS drop because the healer is beating them.
Disc priests just got stuck in a really awkward position, When we heal it's like there are 4 DPS and when we DPS the healer becomes a 5th wheel unless they also go DPS. Really a disc healer is like having 5 DPS at lower levels because tanks usually beat out most the DPS for damage. It all balances out for the end game from what I've heard/read, but while leveling it's pretty overpowered. Really any group without a disc healer is at a disadvantage and for purposes of min/maxing disc is hands down the best spec for regular dungeons.
I guess I am a little greedy for checking DPS to occasionally shave that little bit of time off my dungeon queue, but if I can do my job as good or better than the rest of my group and on top of that indirectly keep everyone healed up, how is that my problem? When I first got atonement I thought how crazy overpowered it was as I smite healed my way through dungeon after dungeon while topping the DPS meters by a fairly large margin. I told myself this can't possibly last as I continued to level only having to actually heal when I got into a dungeon slightly over my level and even then only a few times total. Then they buffed atonement to work with holy fire as well and I got even better. My relative damage has tapered off a bit, but I can still generally get first or second for damage on boss fights. We'll see what happens once I hit outlands, but the way things are going it looks as if DiscPS will remain not only viable, but flourish.
In fact when I was leveling up my warlock I had a disc priest not only heal and DPS, upper blackrock spire, they tanked it as well. They had obviously queued with the tank and the pally tank hung back and healed while that disc priest tanked everything and held decent aggro as well all things considered. I'd seen tanking classes tank worse. I get all giddy with excitement thinking that I will still have that much power in 10 more levels. I may step on healers toes without hesitation, but I still usually let the tank do their job for the most part. I've always been known to pull as a healer when my tank is going slower than I would like and being a disc priest has certainly only encouraged that behavior. I remember I did cause a tank to drop for doing that in Uldaman. I just continued to tank/heal/DPS until we got another tank from the queue (that next tank did a much better job).
There certainly is a lot of leeway in a regular dungeon for what a good player can get away with. On one hand it is kind of stupid that tanks and healers can be made obsolete without any real consequences, but on the other that is the price we pay for having content tuned toward learning how to play instead of tuned to be challenging like heroic or raid content. I remember fondly running dungeons in outlands with 5 death knights before the dungeon finder.
Obviously the point of all this is that you shouldn't be discouraged or upset by someone doing better than you, or even outright carrying you, in a learning environment such as regular dungeons are. If you are the tank and people are tanking for you without any ill effects then just use the time to practice. It's a no pressure environment, you can try pretty much anything you want and it won't matter. If you are the healer and no healing is needed then work on seeing how much damage you can do or perhaps even flip specs and heal from a DPS spec as needed. Healers are given the opportunity to DPS more often than you'd think so getting some practice with it isn't a bad thing. Regular dungeons are just as much learning to deal with the strengths and weaknesses of other players in your group as they are about how well you can perform at your role individually.
I just get so excited thinking about my disc priest, they are like the caster healer/DPS equivalent of the melee tank/DPS hybrid role that feral druids fill. Not quite as flushed out on the DPS side, but it's as close as any other class/spec comes to filling two roles.