Personally, I like it because it limits column width which makes it easier for me to read text.
You actually can switch to a single-column layout in Plus under the "More" menu. It still limits column width, however, and when I've pointed that out to people that complain about multi-column, the response has generally been "but then you have so much wasted space!"
Seems like a damned if you do, damned if you don't situation to me.
This seems to me like a classic case of designers thinking of visual "prettiness" first and actual utility second.
Facebook did this for a while with Timeline, until they fixed it with the newer Timeline which keeps the posts to one column. It really looks like Google+ is just ape-ing Facebook design, even when the design has implicitly been admitted to have been a bad idea.
I am willing to be persuaded otherwise, though, if someone wants to make the case.