Let's pretend that we had the old rules and could have 40 players on an active roster in September and that MiLB was over. No, having Groome on this roster for a few weeks wouldn't hurt him. He might not pitch all that much, but it could be good to have him around some of the MLB guys (he did spend considerable time working out with Sale in the past). I wouldn't expect him to pitch very well in a game though.
In that respect, were MLB GM's resistant to call up guys for the 40 man active roster back a few years ago? If those guys failed, was it rushing them? Did that hurt their confidence? If it would hurt their confidence, a GM would never call up any decent prospect in the old 40 man days.