The disadvantages are obvious, though: ballistics, as already pointed out, and the sheer weight. Not that much of an issue in a C152 or anything heavier, perhaps; but a real concern in a 450 kg gross ultralight.
As someone already stated in some other topic with S-mode transponders in gliders: the extra pound of any equipment is ALWAYS a problem with gliders and ultralights, but nobody seems to lose some pounds of their own weight intentionally just to increase aircraft performance
My opinion: the fire extinguisher is probably the most important part of emergency equipment in an aircraft, next to serviceable VHF radio. ELTs with today's radio coverage are baseless, especially if flying in normally-populated countryside and they won't do you much good unless you are either commited to forced landing or already crashed. Especially in microlights, where the electrical equipment doesn't need to be TSO, I'd say there is larger risk of electrical fire - and what will you do if you have a small electrical fire and you've smartly decided to fly without fire extinguisher on board? Watch it to spread while continuing to an airfield 30 NM away?