At the risk of agreeing with cps (yikes), I have to say that I hate spending my hard-earned money on a game system only to be told by that game system "Eh, we couldn't be bothered to create rules for this particular situation so... just make it up yourself or whatever." By that logic, why not just save my money and make up my own game from scratch?
Don't get me wrong, cps is totally right in that the system being easy to houserule isn't an excuse for actual rules being weak.
But sooner or later, most people I know like leaving their mark on the game they're playing by introducing custom stuff into it, whether it's a unique item, or a modified homeworld so Johnny can play that Catachan Guardsman the way he envisions it, or an awesome new spell for the team's sorcerer, or, heck, just a homemade xeno race as the campaign's BBEG. At this point, "easy to houserule" being in the system's description is a veritable godsend. It may seem like a no-brainer, but trust me, I've seen systems where creating custom content that doesn't accidentally break the metagame in half is part-chore, part-sorcery.
Hence, what I actually want are crisp, sturdy rules that'll both work by themselves and survive whatever ridiculous homebrew I throw at them.