I have a Sherco 2.9 trials bike. It is fun to ride on trails. A seat would be nice. I can pack extra fuel. One time my 12 year old son completed a 62 mile trail ride without a seat.
However, a fundamental limitation of a trials bike is the radiator cooling capacity. Even with the cooling fan running on long mountain trail climbs the Sherco overheats carrying me (270 lb geared up). It works fine for my son at about 110 lb ready to ride.
As a note, my GasGas motorcyles do not come close to overheating on the same climbs.