As to "following the rules" -- I like the "rules-y" nature of some of the MM stuff because it gives a frame of reference to start from.
Then when I want to
break the rules (e.g., "these axiomatic orcs have +2 to Will saves and firearms proficiency, and no smite ability" or "Eh, these guys need higher skills. Racial skill bonus time!"), I do so with some idea of what I'm doing and why. (Call it "three-quarters assed" rather than "half-assed"

)
More importantly, it gives other publishers & authors the same frame of reference, so that (hopefully) critters have some consistency or at least explicit acknowledgement of inconsistency. That helps me figure out whether a book I'm spending money on was written carefully or sloppily. I'd rather give my money to the former. I can usually do the latter all by myself -- it's quick and easy; and if I can't, there are plenty of people on the 'net.
Also, I find it tiresome when a game system continually invents new ways to do the samed damn thing ("Oh, gee. A new way to work poisons. Yay."). I like some systemization in my systems, y'know?
Of course, I could probably blame that on all those math and CS courses.
(I think I'll go look forward to Tome of Horrors now.)