For unification of the system and so that no one has to keep buying the rules in a slightly tweaked format? Buying the rules five times over to play what is essentially the same game with new mechanics costs $300 (5 rulesets at $60 each). That's a disgrace. One core book, unified and overhauled mechanics and then you can have.... A single bestiary. A single armaments book. A single setting book for each tree of the game (BC, DH etc), and the books should cost less as a result of losing all of those pages of rules that were already bought.
Aside from anything, the system is showing its age - compare DH to BC. Time for an overhaul.
I can (mostly) agree with this. Compared to some people here in this thread I have no problem buying one more core rulebook mostly because I left out DW and because I buy only very few non-core rulebooks (usually the advanced players guide and that's about it).
Unrelated to actually buying the 5th core ruleset I hope that this will be the last core rulebook before a major overhaul both in mechanics and in printing / distribution policy. As people have pointed out core rulebooks do sell better but I hope that FFG can switch to a format that doesn't involve repriting large parts of the mechanics in various books.
Maybe a D&D style approach would work for the games with multiple smaller core rulebooks (similar to a players guide, dungeon masters guide and monster manual) and "setting" books that provide you with what you need to have if you want to run a Space Marine themed campaign similar to what WotC does with books like Setting-A Players Guide and Setting-A DM Guide.