By scraping all Google+ profiles, it would not solve the problem of fake profiles. Anyone could use your Twitter username and let your followers think they are you. With migratus, you are sure that the Google+ profile it gives you is the one provided by the Twitter user.

If they can somehow create content that fools me into thinking they are the original person, I might just be as interested in following the impostor instead.

How large of a problem do you think this is?

I don't think it's a problem at the moment, but it could easily become one when such tools become available to the public.

Though, I still think that an non-perfect tool as described above would bring much more value than what migrat.us brings with its 500~ users.