The defensive rating with I.T. on the floor is a team worst 101.2. With I.T. off the floor it is a team best 94.7. He is a tremendous defensive liability. He ideally should not be starting and would indeed be much more effective off the bench.
Thomas has a defensive Real Plus Minus of -2.37 which ranks 65th out of 84 at the PG spot.
He also has an offensive Real Plus Minus of +4.49 which ranks 6th out of 84 at the PG spot.
Combine the two and he has an overall Real Plus Minus of +2.12 which ranks 9th among all NBA PG's.
He might be a defensive liability (since he makes us worse on defense), but his defense is not a liability (since his offensive benefits far outweight his defensive weaknesses).
We are better with him on the court than off - a lot better.
Kyle Lowry is a better player though.
Still, I think we need scoring more than we need defense, since we have a lot of good defenders as it is but we really have no explosive scorers outside of Thomas. If we lose Thomas, then I don't think we could make up for that offensive loss with any other PG outside of Curry or Westbrook. Not sure any of those other PG's can carry the scoring load the way Thomas can.