I think Rob is right when he says that practically no voters can what Jeb Bush tells them to do. There were never many Jeb voters to begin with, and surely not many who might still be deciding between Trump and Cruz were sitting around waiting for marching orders from Jeb.
But I don't think Jeb's endorsement of Cruz was really about persuading voters. It was about giving the signal to establishment campaign donors that - however much they may not like it - party regulars know their only hope of beating Trump at this point is to get behind Cruz. And Cruz is going to need more money than he's got right now, which will have to come from the same people who used to eschew Cruz largely at the direction of Team Bush.