It’s like an honour thing, isn’t it? They’re part of the Doctor Who family. I’d guess most of them don’t act much these days, but still great to see them. If only Nicola Bryant had turned up…
There's not a lot of the classic series companions who went on to have even middling acting careers really, are there? I don't think a lot of them were hired for their talent, more so that there was 'something for dad' on screen, certainly from the 70s onward... Off the top of my head, I think I only ever saw Louise Jameson - Leela, opposite Tom Baker - in anything else of any note (Eastenders).
I didn't mind this special at all, even though the 'big enemy team up' stories tend to be underwhelming. I enjoyed the 5th, 6th, 7th and 8th Doctors finally getting some official screen time in the new series, although why they all look older when the 1st Doctor is the 'same age' will have to be one of those Who mysteries.
Given that they've established it with the 1st Doctor, I wouldn't mind them recasting the 2nd and 3rd Doctors now, either. I guess you could just throw Sean Pertwee in there if he wanted to do it, and there are enough Troughtons about, one of them must have a resemblance.