This is PURE marketing.
1) he wont lose anyone from the ticket because of her lack of experience (Because of Obama is even worse, and hes running for pres)
2) she has an appeal to women (old and young) who are truly on the fence
3) she is conservative (fiscally and socially)... so she is not a smack in the face for the party (unlike lieberman)
4) shell help with Alaska. (Battleground state)