This is not what I'm suggesting. What I'm suggesting is playing your PSP games on your PSP without being bothered by Vita transferring whatsoever and purchasing PSP games which you do not own on PSN. Backwards compatibility is a fun addition, not a deal breaker. The PSP is backwards compatible with selected PSOne titles, yet I don't remember anyone whining that it doesn't have a CD disc drive nor do I remember any program of transferring PSOne titles from CD's onto the PSP.
The route of logic here is clear - if someone has a collection of UMD's that he or she wants to transfer then the likelyhood of him/her owning a PSP is high. If he or she already owns a PSP then transferring the titles isn't really a necessity for him/her. If he or she doesn't then he/she will not be remotely interested in transferring something he or she doesn't even own. You get my drift?
Would it be a good addition to the system? Yes. Is it a deal breaker? No. The Vita was designed with Vita games in mind, the PSP classics are only a bit of icing on the top for those who like to go a bit retro from time to time.