It makes sense, considering what we know from the last episode. Spoilers in invisi-text:
We might get to see River's life from her perspective, meeting up with the Doctor in his different incarnations. We know she's met the Tennant Doctor so why not one or two of the others before him?
That's pretty costly, considering they'd have to recreate the previous Tardis interiors again.
No doubt they've got DT's hidden in a warehouse somewhere (in fact didn't we see it a few weeks ago?), but have they got PD's version still hanging around.