On top of that, while most responses are personalized, the speech has to stick to the general party platform. That’s not necessarily going to be in the comfort zone for every speaker.
Consequently, most responses get lousy reviews, which can’t be very good for the politicians who gives them.
The few that are well-received, however…it’s hard to see help much, either. To be sure, it can help an obscure politician get noticed by the national press, but then again not all that many obscure politicians get to give the response. If it’s someone such as Rubio — who already had a major speech with plenty of coverage this year — that doesn’t really apply.
And since so many SOTU responses go badly, there’s not even the default reason for doing it — that a senator who has the opportunity might as well do it, because if not someone else will.
I suppose there’s always the temptation for any politician to believe that he or she is simply a better speaker than others, and therefore this one will go well. But it’s not about the people involved; it’s the format. If I were advising Rubio, I would have told him to just pass on this assignment. There’s just nothing in it for him.