Multiple attacks in one day can and do happen. A couple of weeks ago, we had 3 in a row; some of us teleported directly from one attack to the next.
There was a horrendous, wonderful time in glitch history, back in late November/early December, when we experienced a 2-hour period in which we had an average of about 6 attacks happening at a time. I'm not talking 6 attacks over 2 hours, here--we were fighting multiple attacks at a time for 2 hours straight. We ran out of map teleports, energy, items to donate...and then the attacks ended at exactly the moment that new street projects began.
So an increase in frequency could be a harbinger of change on the horizon.
They're to distract us from whatever new thing the Giants are imagining up...although I think 'tis pretty darn obvious that there are big changes happening right now, what with the strange performance tests and all.