That is absolutely true. However, provided you have all the buffs needed, then you can replace pretty much every other class with locks. However that's not how it works in most guilds, since you usually simply don't have 15 of one class (remember how even the top 10 guilds struggled to find enough arcane mages and rogues for spine hc pre nerf). Throughout tiers you need a variety of classes and specs, and that is exactly my point: there are plenty of top 50 guilds who had an ele shaman in most of their progress kills. Yes ele is weak(er), but it's not like a well played ele can never get a raid spot. There are reasons enough why one would chose for an ele over stacking something else, not in the least the off healing, a self ress, purge glyph, cleave, burst .... And if that ele is capable he will pull his weight.
I see that every week myself. Boomkins are under the radar, but capable players do sick dmg and healing as boomkin. If would chose any class to stack for my progress raid it would be boomkins (symbiosis is godly, tranq, innervate, combat rez, sick burst, sick dots, sick aoe).
That'd be a very welcome change.