Am I correct in my thinking that glue fillets are a simple way of building a mechanical mechanism to keep a butt joint in place?

Be forewarned that Titebond fillets dry to a dark amber color. If you are planning a transparent covering consider something else.

For what it's worth a local building guru uses aliphatic resin (Titebond original is his preference), a wet cotton swab to clean off all glue residue and finally thin CA on the inside corner of all the joints. Yea, he's pretty anal and goes to the extreme of cutting a dado joint in lieu of butt joints. All of his coverings are transparent - just to show off his craftsmanship. And how much time and patience do YOU have?