I asked Bo the same thing.... I have XS "Small Dot" sights on one of my EDC rotation guns, and had another gun (S&W CS45) with 'em, too. So finding that the M&P version was a tad cheaper got my attention
Same answer from Bo - no place to put the vial.... You'd think they'd put a round vial into the thing, towards the front sight, instead of trying to use the vertical style, but.... IAC, it shoots fine. Not a paper puncher, but quite good enough for SD use. (The "Small Dot" version is only slightly worse - for me - for paper punching than the stock sights on the 1911. Doesn't seem to muck up SD-style shooting.)
I keep flip-flopping about night sights and lasers.... Both of my EDC 1911's have 'em, and my M&P9C ("around the house gun") has night sights, but I'm not sure....
(I love the laser for training, but it is kind of expensive.)