It could just be a ploy to stop the endless whingeing about Tennant not being The Doctor anymore! Every post-reboot Doctor since Tennant has been unfavourably compared to Ten, just as every pre-reboot Doctor since Tom Bakers' Four was in bius shadow.

Give the fan-favourite another bash at it and a happy ending and maybe the new guy gets a fair assessment from the fans?

Anyway, this is now the Whoniverse, so what's the big deal about it? The occasional Tennant/Tate special, just or fun and separate from the overall story arc would be a good addition to the toolbox. Apart from anything else, RTD can have them on standby for the inevitable moment when The Doctor must make their bow in the MCU.

The fact is that all fifteen Doctors are racketing about the Universe all the time anyway. It's only the TARDIS and the general vastness of Time and Space that stop them running into each other more often, making sure they're seldom in the same place at the same time or at the same time in the same place.

As to the final confrontation, the Toy Maker is an incredibly powerful being, so two Doctors were necessary. Catch is probably one of the first games humans played, and like all basic simple games, it's one of the most difficult to win, if you don't cheat.