[quote=@MelonHead]
<Snipped quote by ImportantNobody>

If a transformation is wholly positive it doesn't make sense from a story sense, nor is it really suitable for Arena. 
[/quote]

Even dragon ball z transformations aren't wholy positive, requiring concentration and energy to keep up so they would tire out without practicing it, although admittedly way more beneficial then hurtful.

Of course I would have additional weaknesses as well as the weakness of having to transform to match the tough opponents. The only way I see transformations being horrible for the enemy is if they BS stuff, which they could do with or without transformations, or it's a close match so tey could fight untransformed for a bit as then transform to beat them, but I have no clue why the opponent wouldn't specify if they can transform or not before the match, knowing they couldn't fight the transformed form and only want to fight untransformed.