Why?All the top apps are cross platform compatible
Well for starters you will end up paying for the same apps twice....
Not just that, say you use google music and buy music from their google store, you can only play that on the google music app (or from the web browser on a pc)... so you can't get that on your ipad same with google movies (and vice versa for itunes and apple movies)....
Plus if you have all the same devices, you will not have to sync your stuff twice (contacts/calendars/emails/messages, etc)
(If you have both platforms you will have to sync once through itunes/icloud and once through Google)