If we're talking about Rielly and his attributes as a leader and standing up for his teammates etc. then sure he definitely is valuable to the team, but if we're talking about his actual play on the ice then that's somewhat of a different story. No doubt he plays hard and gives it his all and while he's good not great offensively, he's about average and at times below average defensively.
I've watched Rielly since the time he was drafted by the Leafs, then played his first game with the team and have watched him play pretty much his entire career. Over the years I'm sure everyone hoped he would turn into an elite Dman who could provide offensive and play very good/great defense and be the Leafs #1 guy. Unfortunately that hasn't quite happened and while playing number one Dman minutes, he probably should be playing a few less as a second pairing guy. However considering the Leafs didn't have anyone better, he because the default top guy.
For me I think perhaps I was looking for Rielly to have a Liljegren type of progression. In his first season where he got significant games with the Leafs he started off abit rocky and was making some mistakes and was giving up the puck carelessly a few times, but as the season went on he got progressively better and by the end of the season he was playing quite good. This season he's stepped it up and has been perhaps the Leafs most steady and reliable Dman who you can trust to play the most difficult of defensive situations. That's what I hoped Rielly would become on the defensive side, but it hasn't ever happened to date.
Who knows maybe there's still room for improvement for Rielly, but it seems unlikely at this point in his career. Perhaps the best you can hope for is to have Liljegren play with him and see if that doesn't work out. As for Rielly getting traded maybe before he signed his new contract there was a half decent chance of that happening, but now it seems highly unlikely at least for a few seasons unless something drastically changes.
Personally I wouldn't mind if he were traded and think it would hurt the team more in the locker room than on the ice. His cap space could be used to resign some guys like Bunting and maybe add another forward. Offensively I think Liljegren, Sandin and Timmins can make up for the loss of his points. I mean Rielly is good offensively, but he's not Makar or Karlsson level of being an elite point producing defenseman.
Lets hope when he comes back he can bring it up another level, but if you look back at some old threads on this board, you'd see that more than a few people have talked about his defense for a number of years now and that this wasn't just a recent thing.