I am not so sure about him being a bust. Look at his 2002-2004 numbers. 2002 looks about what you would expect for a rookie forced to start 12 games for a bad team (215/429 for 2294 yds, 12 TD, 16 INT). In 2003 he started a full 16 games for a bad team, but improved his performance to 309/554 (3rd most attempts in the NFL) for 2880 yds with 17TD and 22 INT. In 2004 he started all 16 games and improved to 277/489 for 3047 yds with 19 TD and 12 INT. Not bad improvement for a 3 year span on a bad team.
Then in 2005, the team was in shambles. Rogers was suspended for 4 games, Roy Williams was in and out with leg injuries, their running game took a hit with Jones battling injuries, Mooch was fired as head coach during the season, there was a QB controversy with all-pro-Pro-Bowl Backup Garcia, and lost WR Kevin Johnson for the season in week 7. Oh, and Harringtons stats suffered a bit, 12 TD 12 INT and 188/330 for 2012 yds in 11 starts (his completion % improved, however).
He was on a nice trend of improving until the disaster that was the 2005 Lions football season. I am not sure you can pin all of that on Joey's shoulders. The Lions fans will tell you that he is tough as nails, but they are concerned about his ability to lead the team.
He is a 27 year old 6-4 220# former 1st round (3rd overall) pick. His character is solid. He is tough. He likely will not get a starting job this offseason, as the market has been diluted with Brees and Culpepper, and the next three teams needing a QB have good prospects for the draft.
Could be the kid just needs a fresh start with a winning organization. I am not saying Dallas should hand him the keys, but why not consider him as backup material?