It won't hurt.
The good thing is that it "guards" little things that the others can't. If you are not into PC maintenance and want to leave it up to software to "cover your kiester," then it's good, I think. To those I would recommend installing it and turning every option in it on, so that it's active all the time. To those who use IE I doubly recommend it. (I find it ironic that it's mainly to protect those who use the browser of the same company because that's how most of the bugs it finds get in.)
If you like doing your own PC maintenance, know what you're doing and are careful, I would install it to run manually once in a while, but leave it "inactive."
If you use Firefox like me, it will have very little to do. Neither it nor Adaware has found any spyware or bug after using only Firefox. Twice, and only twice, MSAS found something after a browsing session with IE, and each time it was only one minor thing it found. Also, running Adaware directly after MSAS after an IE session has found a bug that MSAS didn't find, while the reverse has always been negative.
So, my own conclusion for Firefox users is:
MSAS does watch a few things that other spyware programs may not, but it will most likely be unnecessary in a PC where the user uses Firefox exclusively. It won't hurt to have for sessions where IE is necessary, but Adaware seems to find things it ignores anyway.
As long as it's freeware, it won't hurt to keep testing it.