He didn't quit. He did leave the field once but there's more to that story than him just getting in a huff and walking away.
In his own words that were not contradicted by Kiffin, Poole said he often confronted CLK about when he was going to get his chance. He basically challenged CLK with the FACT that he was no worse than the 2nd best back consistently in practice. According to him, CLK never denied what he said but just shrugged it off.
That definitely sounds like there was a recruiting angle to the PT decision.
Then... Poole did go to CLK to discuss leaving if he weren't going to get a shot. I've never seen anyone say what was said and promised... but Poole stayed. That leads me to believe that CLK acknowledged that Poole should have gotten a shot and promised him a fair one.