Let me join the fray here.
Grew up in the country and rode bikes any time I was not doing the chores on a large farm. I had a CT 70 and an XR 75 when young, I used to be intrigued with trials and would make little routes up hills and through creeks on the farm and try to ride them without putting my feet down. I sold a show calf one year and earned enough to buy a new bike, I was 12 or 13 I think, I wanted to get a new TL125 because they seemed to be the hot setup in all the motorcycle magazines I read at the time. I went to the local Honda dealer, not only did he not have any TL bikes, he seemed to insinuate I was a bit "slow" for even wanting one and did not appear to eager to order one for me. Over to the Yamaha shop we went because my second choice was an IT175. Well right next to the IT 175 was a YZ125D. It was a little taller, and a ton more sexy looking and was the exact same price. I jumped on the YZ125D and told my step father this is one. I got that thing home and literally crapped myself at how the front tire would head toward the moon whenever I hit the powerband in any gear. Within a short period of time me and that bike were like one, I rode it all over our 860 acres, wore out back tires and rear fenders with abandon and spent my entire youth of motorcycling into MX. Yes it was big step when I took it, but it was very quickly I realized that I must respect the bike, once I learned to respect it much was learned very fast and a ton of fun was had.
I just bought a used 2001 Gas Gas 321 TXT a few months ago after having not owned any dirt bikes since I was 18 (30 years ago) and moved off the farm. It runs fine and is I presume is like most trials bikes, a torquey motor that makes power at low RPMs, the exact opposite of a Mxer. I am sure there are modern bikes that have a powerband that hit much harder than this model, but I am damn sure they are nothing like a brand new YZ125D. I see no reason any one familar with motorcycles should shy away from a bike that fits them in order to learn a few new tricks regarding trials. I haven't been on a dirt bike in 3 decades but have had no problem in controlling this bike on the trails and on the steep slopes in my backyard, on small obstacles in my yard and through the creek behind my 5 acres. I look forward to progressing to skills that would enable me to clear more difficult obstacles, but I am not interested in winning trophies or impressing others, I just enjoy getting some exercise in a way a ton more fun than a gym.
The key is respect. If you acquire a bike for something that you are not experienced in, you better have some respect for that is which is unfamilar to you. Trying to protect yourself from lack of respect by getting a bike way to small for you is not the correct idea in my mind. A lack of respect for any bike you are not familar with is a sure way to get another lesson in why Mr. Gravity is undefeated.