There is also the fact that a well geared red line LM will do more burst aoe dps than a champ easily which is all the trash is about in T2. We often take two LMs and go dps for the boss taking turns for the debuffs etc.
We started this to train other LMs on the CC and found out that no champ is not such a big deal especially in Lightning, the LM stuns the crawler spawns then aoe's them down. Personally with the Draigoch set on (don't use macros) have had ents crits not devastates on 4 targets of over 14k......., add lightning storm and a couple of sticky gourd's and cracked earths, + all the usual support stuff (defubbs heals pwr share etc) and you have a pretty capable damage class so why not use it.
Also I'm probably wrong but isn't stacking a certain class to get something done easily a bit boring and takes the fun out of it.
We don't take two Lms all the time but if two mains sign up then there is no harm it works well for us anyway.