The question is whether or not users want to be forced to use the word as their folder name. While a very clever way to add in plus addressing with almost no UI change, this would block a few things:
You can't change the folder redirection once the keyword has been selected without making up a new address.
IHateYourCrap.spamcowboyATspamgourmet.com -> hoss+NotCrappyAnymoreATexample.com
You can't redirect several lists into the same folder.
d'etroiteNews.spamcowboyATspamgourmet.com -> hoss+newsATexample.com
undernews.spamcowboyATspamgourmet.com -> hoss+newsATexample.com
You can't send to a subfolder, at least not with any address format already described.
SGList.spamcowboyATspamgourmet.com -> hoss+lists.SpamGourmetATexample.com
However you might choose to interpret lists.SGList.1.userATspamgourmet.com, you cannot simultaneously support both the above features.
Your simplified techinque is based on the idea that SG addresses tend to be remade constantly so if you want something filed to a different folder, make up another address. You think primarily in basic mode, which without any controls, it's really a stay-out-of-advanced-mode mode. This is fine if SpamGourmet is only a stepping stone by which partly trusted senders graduate to fully trusted sender at which time they are subscribed with a direct address.
Since I consider all senders as untrusted forever with the only variant being the size of the distrust vector, my technique sees SG as a permanent link between me and those with the greatest ChangeDifficulty*Distrust vectors where links are persistent as long as spam does not interfere with the desired messages. SG allows me to specify the Terms and Conditions my for my address and then terminate the contract when use or misuse breaks it. I'm assuming that exclusive senders continue to forward after the counter hits 0. Basic mode is too much busywork creating new addresses constantly. Advanced mode is set-it-and-forget-it until a problem arises.
Spamgourmet as an interlink allows me to change email providers and hundreds of occasional use forwarding accounts follow me. I can terminate any single spam source with little inconvience. With Plus Addressing, it can also be my no-errors filing clerk that I can rearrange any time. Anything that generates serious volume, attachments, or two way traffic I generate an alias for rather than running that traffic through SG.