I'd say that binary packages are really valuable for installing a global haskell dev environment for quick hacks and scripts for which it would not be practical to setup a project and download and build a lot of deps. At least ghc and base should be installed globally so one can just fire up the REPL to try things interactively.
I have noticed that community has gotten real good support for Haskell (again), and I have switched to using that because I prefer community controlled packages. For me this has worked out without any problem (I used arch haskell before), so there may not be the need for arch haskell at this point...
I am very greatful for all the work you've made on the arch haskell repo. It served me well for a long time when the official haskell support failed.
Regards
Johan