I guess it has its practical reasons - they can do the jury votes while the televotes are being counted, but on the other hand, if all the jury members are there watching the shows, surely they can get their votes in while the televoting happens?

You can't exactly go 'okay the juries have voted and these thirteen acts aren't in with a chance anymore so you can't vote for them' either since then what would be the point of having them perform. Besides, it would mean missing out on things like Germany getting zero points from the televotes :p.

Best solution would obviously be the jury voting for the final performance and letting the rehearsal actually be a rehearsal!