It's a good point. Long term benefits of imports? Like most guys you pay, they'll be there each cricket season, when/if the money dries up obviously they'll go (particularly the imports but a lot of guys recruited from outside the comp would be the same). Its up to the clubs to look at there individual positions and lists (which they know better than anyone else) and decide if that's the route they want to go down.
I like the imports, playing against them is always a good challenge and they tend not to sledge and carry on like some of the ego's going around, well not anything I'm in danger of understanding.
I don't think a cap on overseas players is a solution though. Clubs will still get players, they'll just be the hit and miss ex-district type we're more used to seeing. Unless you consider a comprehensive player points system then leave it as is.
Any club looking for war silver bullet solution to win a flag with imports, without putting the necessary time into their juniors will be shooting themselves in the foot.