Not a bad point.
Another good option is a dirt jumper frame. Not perfect on geometry, but better, and generally has shorter dropouts. Far more available than Trials frames.
That said, you can have quite a bit of fun on a Mountain bike with good brakes. I run AVID 8" mechanical disks on my Moustache Cycles Odin All Mountain Bike, and find them pretty good for trialsin around. My trials bike runs hydraulic disks, but they require more maintenance and are not as strong all around. I ran the Magura Rim Hydraulics for years, and there are things to recommend about them, but the maintenance is high and can be frustrating. Depending on how big you are, buy some heavy duty wheels now. I walked home too many times with destroyed wheels in my first year.
The main point is to tear into it and have fun. If you ride much you will destroy whatever you start on, and can look for something better.
ATGATT: When you fall off your motorcycle, you will be wearing what you were when you got on your motorcycle. Dress accordingly.