I thought it was stupid looking until I tried it. Carbines with A1 stocks are very comfortable - very ergonomic, and you are only giving up a couple inches from totally collapsed. It also doesn't grab your beard, and as was mentioned, the rifle buffer is about the same weight as a 9mm buffer, and we know heavier buffers in short carbines are generally good. I haven't shot it yet, but I am running my A2/M4 on a fixed stock and love it. I have to say, I'm guessing the real reason for the XM177 pics you see was just trying to make the thing run consistently, since they didn't have heavy carbine buffers then. The 651/652 has a wonderful feel to it - only thing better might be a middy. After handling fixed stock carbines, I'm totally sold.
ETA: Anyone notice that the mag in the top photo Hal posted is an early fully-curved 30 rnd? I've seen that pic dozens of times and never noticed...