I, like most 40k players I'm sure, feels the need to collect more and more armies. Oh, what if I do this or I really like that model, I want to try that paint technique... there are lots of reasons. But I find that CSMs are such a versatile army that I never need to leave my codex to try something out. I am really happy that I chose CSMs as my first army, because hopefully it will also be my last. Glory to the chaos gods!