Your sacrificing your overpowered thorn/last word/messanger(thats all people ever use in trials) in exchange for a slightly overpowered heavy. It not a cheese its just an option you have. Your losing out on the ability to knock people off of your/their heavy with your op primary in exchange for having a better shot of taking them out once you get heavy. I've seen both the enemy team and some of my friends do it and both have gotten mixed results.