well, the advantages of a 4 track looper are similar to a 4 track recorder... obviously, with a 4 (or multi) track recorder, you have access to different parts of your mix.. instead of the whole thing on a stereo channel.. so you can turn different elements up and down.
same with the 2880.. put some sounds on one track, others on another... and now you have independant volume and pan control for different elements in your loop.
the looper on the El Cap isn't so much a "looper" as it is like repeating part of the tape recording head.. i mean, it *is* a looper... the weird part is how much it degenerates with each pass of the loop... what you record in the loop quickly becomes a washed out blur.
the thing with the El Cap loop is that it is constantly re-recording itself.. every pass.. so if you have any kind of tape loss, wow and flutter, etc, dialed in, it will progressively get more and more pronounced until it's a big soup....