I think that it all depends on how often a segmail address is compromised and needs to be rotated. Intuitively, and from speaking to people that use a manual system in the spirit of segmail - this happens, but quite infrequently.

I think the majority of spam comes "over time" as your address is copied/sold from one spammers address book to the next.

I've definately noticed that my older addresses get much more spam than my newer ones. I think this is evidence of that fact.

-Andrew.