if a team has an old 3* player who cannot gain anymore but still playing good, it's ok.
But I don't see any reason why someone to buy a 3* fast trainer. If he was 4*, maybe yes (if you have a lot of greens and there is no one 5* player in TM at that time).
A 3* or 4* or 5*, fast or slow, cheap or expensive, has the same possibilities to perform good or bad in a new team.
so, as they say, better be wealthy and healthy ...