I think you're absolutely right that he was hastily put into the series.
It could have worked, all the best comedies have a shifty wheeler dealer type with the gift of the gab.
But this set up was gradually removed from the show, presumably to remove Tom from being part of the trio, and to make way for Billy Ingleton's increasing appearances.
It's often been said the age difference with Tom's character wasn't right to be with Clegg and Truly.
It could though be said that Tom was the nearest you could get to Compo, which in many ways is very true.
I thought Tom's acting ability was ok when he had a chance to use it, though in his later series he brought nothing to the show.
I think there are many people who thought Tom was better as a minor character, though I can't see it myself.
I think he had a very difficult, uneviable task to perform when he first joined the show.
All new characters take a little while for the actor to get right, a task made all the more harder if the writer and director start changing things.
He was hardly a big name when he joined the show, apart from being the son of Bill Owen, so I can only guess why he was reportedly disliked by some cast members.