I have done it back in the day. It’s a real hassle. And I went to the trouble of designing a proper heating and aligning tool to do the job. I could produce a smooth and pretty good splice. The trouble with it was different brands/colors of filament were always iffy about staying welded due to the additives. While it mostly worked, the welds could break just often enough to piss you off. I finally gave up on trying to make a reliable product happen.

Automatically switching spools on runout is the one thing my AMS lite is really, really worth having. It’s why I bought my Mini combo a year ago.

In my makerspace, we’ve printed using the ends of rolls just to make random multicolor prints. I also recently bought a cheap 3D printing pen for repairing prints or other random purposes, so that’s also a good use for the ends of a roll.

if I want to torture myself and, hum, make a custom multicolor filament, yeah.

I prefer to respect myself, so I have a sign saying “Don’t” on the box of junk with the splicer and similarly bad ideas.

1m of filament represents 6-10¢ of value for basic filaments like PLA/PETG/ABS/ASA from a quality manufacturer, less for cheap ones. I will never bother with the hassle of splicing filament pieces for that.

Small pieces of filament are good for a 3D pen if you have one. Splicing filament isn’t really worth the effort just to use up such a small amount of filament.

MMU units like the ERCF allow you to use a functionality called “endless spool”, which detects spool run-outs and automatically switches to another spool in the same group.
You still waste some filament, to be precise the length from the MMU unit inlet gates to the extruder, but whatever.

Bambu allows you to automatically switch to a different spool from ams when you run out and continue printing, so this becomes unnecessary.