From a structural standpoint, I'd agree that epoxy is overkill for LPR fillets. That said, I still use them *all* the time. Not because I'm seeking strength, but because they're the fastest, easiest way to get a nice smooth fillet. Most times, they're smooth enough to be painted with no sanding.
Here's my method:
1) support the rocket so it's horizontal and that two fillets from adjacent fins are up (e.g. on a 3 fin rocket, one fin would be hanging straight down on a 4-fin model, the fins make a perfect "X" - each a 45deg)
2) mix a bit of 5-minute epoxy - just enough for the two joints that are now facing "up"
3) apply a small amount to each of the two joints. A little goes a long ways! Remember this is an LPR fillet, not caulk around your bathrooom tub.
4) with a finger (always covered in a vinyl/nitrile/latex glove), dip it into some alcohol to wet it and immediately smooth each joint, leaving just enough to form a nice, small, smooth fillet. It typically takes two or three passes with your finger to get it just right, so be sure to wipe excess epoxy off the your finger and freshly dip in in the alcohol before each smoothing pass.
5) You should have a glass-smooth fillet in each joint. In a few minutes, they will set enough so you can repeat for the remaining fillet "pairs" (remember, a pair are the two joints closest to each other on *adjacent* fins, not the two on the same fin!)
I've done this with 90-second (!) epoxy, too, which makes the process really fast, but I'd recommend practicing with the 5-minute stuff before using the ultra fast stuff - once it sets, if it's not "just right" the only way to clean or smooth it is with lots of sanding.
No sanding, fast, and (obviously) more-than-strong enough. The downside is needing to be extra careful because epoxy demands an extra measure of safety. Some may claim that epoxy is too heavy, but I contend that a good fillet is pretty small for most LPR and it can't be significantly different than a PVA (white/wood glue) fillet in terms of weight.