Some traders use them. I don't use them myself but it wouldn't be bad to see the market breadth to confirm the breakout, etc.
Just starting to read
John Carter's book and he mentions that he uses ER2 as a leading indicator to trade ES, YM, and NQ. I didn't know that myself but worth verifying if it's BS or not. I know many ER2 traders keep an eye on ES to confirm ER2 movements. I don't do that either. I just use ER2 prices and volume. I may start to incorporate the TRIN, but have to observe it first to see if reinforces my method or muck it up.