Kevin Johnson, DC, Marvin Harrison, Bill Hurley, Rony S, Marcus Sales, Rob Carpenter, I could go on but based on being around this program longer than many have been alive those are just a few.
And in the NFL it was Marvin Harrison's work ethic that separated him from most others, his extra work in practice with Manning stood out for years, and is one of the reasons that he and Manning set records in Indy. Physical ability may help you get by against weaker opponents, but if the work ethic isn't there the overall team result won't be as good. Obviouly there are exceptions but for the most part sports is littered with guys that had superior physical gifts but did not have a good work ethic and it eventually catches up to you. It can be a cancer on a team, other guys figure they don't have to work hard either. And just because a guy may make a few plays in a game, alot of times its the little things that go unnoticed by the fans that the coaches see in a game that a player that doesn't work hard in practice may not do that affects the game overall and can lead to losses.
Some coaches may treat players differently in terms of how many mistakes they may allow one player to make vs another, but if you start allowing players who don't work hard in practice to play its a slippery slope to staying bad. Most of the successful coaches won't tolerate it.
I've been in only successful basketball programs during my playing days, many times without always having superior physical talent on the team, but the work ethic and willingness to put the team and winning above individual glory was the key, commitment to success is only words unless its backed up by work. Occasionally a physical talent comes along that in college can make an impact, (Walter Berry at St. Johns in basketball is a prime example, horrible work ethic in practice that Lou put up with, that if you didn't see it you wouldn't have believed it, but he could dominate a college game, unfortunately in the pros it didn't work). And the same thing happens in college, a guy dominates in high school but doesn't work hard enough in practice and while he may have some success, isn't enough to dominate like he might and help the team out more. We don't have those physically dominating players, so if they don't work hard in practice overall they may hurt the team if they aren't doing the little things.
We don't know what goes on in practice or how Ashton is doing, but if he isn't working hard and doing what Marrone expects, I'd rather he try and nip it in the bud now, nothing worse on a team of mediocre talent to allow a player who may or may not be really good, (especially a guy who wants to be the QB) to not put out in practice and hope on faith that he can get it done in the games.