Rodman is one of the best rebounders in the history of the game. He led the NBA in rebounds per game for a record seven consecutive years. His career average of 13.1 rebounds per game is better than Dwight Howard, Charles Barkley, Tim Duncan, etc. He was not just a lock-down defender; he was the best rebounder in the NBA looking back at the last 40 years.
Artest/MWP, for all of his mental issues and off-court (and on-court) shenanigans, is a supreme athlete (with tremendous upper body strength) who was a scorer before he went to the Lakers (he averaged over 18 points per game for the previous seven seasons before joining the Lakers).
Solely being a defender without bringing a professional level skillset in another area is not usually enough. And with the roster size limited to 11 players, there is no room for someone to only be a defender, "irrespective of their offensive contributions."
Regarding your statement that "she can be an effective shooter if she doesn't have to play 35 minutes at the 4 against much bigger players," last year, when she was the de factor four, was actually Kelly's best year
as far as shooting percentages (43.8 percent from the floor, 31.5 percent from three).
Nevertheless, teams do keep certain players on the roster who are only "shooters" who really cannot do much of anything else, as far as a professional skillset is concerned (Laurie Koehn and Erin Thorn come to mind, though Thorn did learn to diversify her game at some ponit with the Liberty). So there is always a chance.