Because, as stated, it depends on the fight and the class/job of the player. Gears is also a factor.

I've seen a tank rage on WP because both DD were "bad", because their DPS was below average (he used a DPS meter apparantly cause he was giving numbers...).
Now, in context :
Tank was I90, I (healer) was I92, DPS were I50 at most, first WP for both of them...
Tank tried to speed run and took 9 mobs together, DPS were a monk and an Invoker...
After the first wipe, he rage quit giving DPS count for each, shouting they were noob... They were noob indeed, but their DPS was OK considering their gear... (in fact tank didn't used his CD well too and couldn't keep aggro away from me, he wasn't better)

And that what kind of problem a DPS meter could cause. It has to be factored by the ilevel or it will be miss-interpreted.