Shadow Priests also take less damage than other casters and we heal ourselves. It takes a while for our dps to start being dps due to nature of our spells. There's a lot of moving in Ulduar, there are fights where you have to stop dpsing.
Ignis - casts Flame Jets - that hurts direct dpsers (mage, destro lock, elemental, balance). Even for a bit, they _have_ to stop while our dots tick.
Deconstructor - we just dps our heart out. VE is up, we almost heal everything that gets damaged where poor mages are left at mercy of healers (along with other 3 casters).
Kologarn - when Eye Beam is cast - we run and still dps. If shit hits the fan, there's Dispersion.
Council - a lot of movement involved, direct dps suffers a lot, a sp
Freya - multidotting some of the adds, able to dps more targets at once (not talking about exploding plants, talking about elementals).
Mimiron - a sp is probably the best there - takes less damage, heal from massive damage we can do (5k dps min) and also - when shit hits the fan we just disperse. I also tanked the aerial unit, I was preferred over a warlock. Yes, I know warrior can spellreflect tank but in TBC it were warlocks who tanked such encounters (Illidan, Twins) - now WE can do it too.
Yogg-Saron - we dispel, we abolish, we do nearly everything on the outside that's needed and we dish out a lot of damage.
When I think about it, the sp is the best caster to play survivability-wise and independency-wise in Ulduar. Yes, a mage and lock can do more damage in theory. That doesn't necessarely mean they do it. An encounter consists of things such as surviving. Unless you live, how can you DPS? We're self-sustained most of the time.
I've tried raiding with mage alt that I have - while numbers are larger, the general feel is utterly bad (not to mention utter boredom from mashing fireball). I wasn't able to save myself except iceblocking or using HS / hp pot when problems occured. The damage isn't everything, there's fun factor too.
SP is probably the most carefree ranged dpser in PvE. When played right, it can prove to be a real dps asset. Also, hitting some large numbers is possible if the player isn't a tool (at gimp fights with ridiculous buffs ofc, on a patchwerk-like fight rogue just annihilates like XT).
A well positioned Mind Sear can do funny things to so-famed damage meter.
As I said, I've tried the mage next to the priest and while it has the potential to do more damage - it doesn't necessarely mean it's more fun or more advanced. In the end, this is a game designed for fun (and earning shitloads of money).
The question is, do you play for fun or to be on top of Recount? I played the whole TBC sitting at the bottom of dps meter (except illidan ) and I never had regrets since I enjoyed playing over measuring dps.