The most common knock against alignment is that it is restrictive ... that it forces every conceivable personality or philosophical outlook into one of nine "compartments."
This is a fundamental misunderstanding of alignment.
Alignment for PCs is descriptive, not prescriptive. Alignment is a reflection of how the PC has behaved in the past; it says nothing -- except in a predictive way -- about how a character must or will behave in the future.
So it isn't that a player or DM has to "force" overall behavior into one of nine alignments ... it's that the nine alignment "compartments," together, are as large as necessary to contain every type of behavior.
Another common mistake people make with alignment is that single acts can cause an "alignment shift." I belive this is because of how alignment affects classes like the paladin, wherein if the PC commits a single evil act, he loses his paladinhood. What people don't seem to get is that "losing one's paladinhood" and "shifting to evil alignment" are not the same thing. While it's true it takes only a single evil act to cause a paladin to fall, that fallen paladin, at least for a time, is almost certainly still Lawful Good in alignment. It takes a pattern of behavior over at least some length of time before alignment will refelect the PC's new outlook.
I like alignment, because as a DM it gives me an immediate "shorthand" on a creature or NPC. Although I would much prefer to have a detailed writeup of personality and motivations, if absolutely necssary I can take an alignment given me and construct motivations and a personality that falls within that alignment.
I also like alignment because it makes possible broad fantastic strokes like a war between Heaven and Hell, with PCs taking sides with holy weapons. I am not a big fan of moral ambiguity in an RPG, unless it's something I deliberately choose to insert for a given purpose. RPGs, like theatre, are best when everything is exaggerated ... once you start mixing black and white, you end up with grey, and once you've got grey, it doesn't matter how much black (or white) you add to it to try and recover contrast ... it stays grey. And, barring very specific circumstances, "nothing but grey" makes for a boring RPG.