Apart from anything else, under the 'real name' policy, it means they now attach your real name to your likes / dislikes, which presumably means they can increase the value of information they give to advertisers for targeting (I'm not saying they sell the names, but the fact they can cross reference you to a real person means they dramatically increase the value of the portfolio of other info they have).

NB: I haven't done the upgrade, but I'm presuming that when you opt into it you come to a page that exhorts you to give them your profile with your 'real' name on there instead of whatever you might have had previously in your Google account.