I have a very simple reason for wanting to pack as many mods in as possible. I like to experiment with different types of machines, etc., and new mods, and I like to play survival. I don't want to get a very good start, which takes hours to weeks to accomplish, on a server, then discover another mod I want to play around with (Thaumcraft, for example, which is rather resource intensive for research) and discover that I need to start all over building up resources on a different server. I also like to incorporate elements from several different mods in my machine rooms and buildings. Sealed doors from Thaumcraft on an engineering block wall from Xycraft? Yes, I can and will do that.
Bottom line: different people have different play styles. Pick a mod pack you like, and don't jump on others because they don't fit your standard.