Anne of DIY & Dragons left a comment on the last session recap mentioning she liked the "body horror" of ASE.
I had actually never thought of it in those terms - although that does tend to be the net result - so I'll describe my "theory" here.
Player characters evolve through play - their stories are the result of actual play sessions - at least they are in MY games. Character backgrounds are boring, it's interactions between people (players and DM) that are interesting.
I put a lot of stuff in ASE to deliberately allow for weird body modifications to enable interesting character evolution. This is a kind of conversation between the DM and the player - who is mostly a willing participant, although not always - that results in the organic growth of character background.
The game itself is a way to generate these stories dynamically (and enable infantile humor at the table, in my campaign). So I make a conscious effort to include the tools to make this happen. For voluntary tools, I leave a LOT of things with obvious consequences around for players to play around with. This creates a sense of discovery and a sense of dire consequence. Character death is certainly a modification, but not super entertaining, so I lean towards body modification where I can. Changing PC capabilities is a lot more interesting for DM and player than reducing their capabilities (via death, stat reduction, etc). I've got that too, players should genuinely worry about consequences, that causes them to weigh their actions - and thus choices are interesting for players - but if something seems like it will do something, and isn't obviously a trap, it won't be a "ha ha gotcha dummy", it'll be stranger than that. Players need to trust that their DM isn't just randomly screwing them over.
[of course, LotFP style adventures take a different tack, and they're a different kind of fun and they work, but they have a much different mood - my players could tell that Death Frost Doom was operating by different, more lethal rules when they went through it and adjusted their behavior accordingly]
Now for involuntary body modification - those tend to be reversible (such as the face stealers) and thus a "plot hook", or just better than dying in a pinch (such as getting operated upon by Dr. Giggles - really, who expects quality medical care from a dungeon clown?). It's still generating story, but it's not quite as awesome as players doing it to themselves - when someone volunteers, they don't know WHAT is going to happen, but they know the consequences are all on them.
In summary: I'll do both, but prefer players to sit in the head exchanger of their own accord.