most "meta" raid groups go with 3 melee and 1 ranged, so all the left over casters and ranged are left to pug, with the melee who coudlnt make it to raid times.
as an example, i know more good melee players all in raid groups, doing all content together, than i know ranged/caster dps. but all the caster dps are always using pugs, and the few melee i know using pugs, arent doing their rotations right, and dont last long in raid groups.
The only reason my RDM is in a raid group, is due to a lack of good melee players being available. When I left my raid groups, they all said they loved me and all, but they really wished i played a melee instead, so I could put out better numbers, and or bring utility. (Non raiders like to think of RDM as the progression DPS, even though its useless to be rezzed up w/o resources, mid mechanic, not really learning anything, as ur no longer doing what you would be doing, in the real thing. essentially learning nothing, while the RDM does no DPS, being out of MP from the rezzes. SMN is far better for their utility, and rezzing once per lucid, is the only acceptable amounts of rezzing in a savage fight honestly.)
I usually act as a melee when our groups are missing a melee, and we pug another caster, but of course we still need at least 1 real melee for the LB.