Kelly absolutely had to come into the squad when fit... Croft just doesn't provide enough attack. Tries hard sure, and realistically he's probably a better defender in terms of solidifying that side when the communication is good... I had already turned off when Kelly was given his cameo so not sure how he went, but I don't recall him being a liability out there in previous games.
We just have so little attack across the team that we can't leave everything on Gamble, especially when Staggs drops out for the year, so we absolutely need Kelly out there to give us more focal points across the park. Kelly also likes to float across either side of the ruck to put questions in the defence's mind.
Funny that you note TC onto the bench and Riki being gassed... when I saw our confirmed bench for the game I thought we were a little light with backrow coverage. Kennedy, Bull and Willison are all middles, and Palasia looked to be our only backrow coverage across the forwards and he was starting at prop... so unless his last stint was going to be a sub on for Riki then he was going to be playing the 80... and we already know that he gets loose in defence when he gets fatigued.