I've found myself taking these line more often lately. What he does is mostly a "scare" bet.. he probably has 'something' and just wants to see if he can maybe fold you out or if you are going to reraise him, in which case he folds. If he gets the impression here that you call b/c you are on a flush draw with one diamond, and no diamond hits on the turn, he might be getting the confidence to raise it. Then you can over-the-top reraise him.. or maybe wait for the river to do that, depending on your read of where he is at, the actual turn card, and basically what you feel like.

My perspective on this matter atleast.