I hope Slush stays at 1MB limit. Otherwise it is time for me to move to the pool that does and respects non-hardforked rules. (Does not matter if it is called "Core" or otherwise.)
But of course people who prefer >1MB branch (and believe that it won't lose its utility and consequently value) are free to commit their hashpower to pool that creates bigblocks.
Problem is that pool operators have not stated their policy clearly (and they are more and more influenced by mass pressures). And even if they did, there is no guarantee they will keep this.