It's really a good idea. They could totally up their FS hours just by being NICE to the people that they already LIKE and approve of. Of course that might help retention in the cult-not a good goal, unless it helped turn the cult into (dare I say it) part of Christianity.
They will say that it is not something that they need to track. And for the WTBTS, it isn't. It isn't revenue producing for them, nor is it bringing anyone new in. But for JWs, it would be wonderful.
One think they COULD consider though is something that marketing/advertising folks know already. It is much cheaper to retain customers than to get new ones. If they could stem the tide with some old fashioned good deeds, you would likely see a numbers shift more dramatic than a new date for armageddon.