I've noticed in road racing and flat track most pros practice on smaller bikes. Valentino Rossi and Marc Marquez are often posting vid of them practicing on 450s. Colin Edwards and his crew are usually on TTRs. A lot of the flat track guys do 450s.
I can see the value in it. You really have to focus on fundamentals and technique. Anybody who rides a 125 will tell you that is the case. It also slows it down and makes it safer.
Do people do this in MX? I mean pros or otherwise? I never see it or hear about it. The difference between a Honda 213V and a CRF 450 seems much greater than a 450 to a 125. Why would it work for that discipline and not MX/SX?