There's an assumption here that doing this would result in a material number of users for Google+. But I'm not sure it would: it's reasonably safe to assume that everyone using Google Reader knows about G+, and is either using it already or has decided to not use it. At best, what's proposed would convert some percentage of that latter group (maybe) over to using G+. It
might be a couple million users if everything plays out well. It's probably less.
That sounds like a lot, but at the scale G+ and FB are competing, a few million (possible) users isn't worth allocating dev resources for. They're looking for things that move tens, or hundreds of millions of users. Some opportunities are just too small to pursue.