It's fashionable to dump on Pioli overvaluing Cassel. But finding a franchise QB is perhaps the hardest and most important goal for any team, and it often requires taking a few chances. Look at the Seahawks: they traded for Charlie Whitehurst, that didn't work out, they signed Matt Flynn, they drafted Russell Wilson, and it looks like they may have gotten lucky on 1 out of the 3.
Cassel wasn't a bad move at trying to find a QB that the Chiefs could win with. In 4 seasons he had one season as a Pro Bowl alternate in which the Chiefs won their division, and he was doing decently last year before hurting his hand and going on IR. He's struggled with a bad OL. But look at the guys taken with earlier draft picks, such as Mark Sanchez and Josh Freeman. And look at all the QBs taken with 1st round picks who have struggled: Alex Smith (#1 in 2005), Vince Young (#3 in 2006), Matt Leinhart (#10 in 2006), Jamarcus Russell (#1 in 2007), Brady Quinn (#22 in 2007), Jake Locker (#8 in 2011), Blaine Gabbert (#10 in 2011), Christian Ponder (#12 in 2011), Brandon Weedon (#22 in 2012).
Hitting on a QB is hard. Teams that don't have one are desperate to get one, and if it takes swinging for the fences a few times its worth it. Who is there in this draft at #6 who is going to transform Cleveland into a contender in the AFC North? They have a solid defense. Adding another pass rusher, or another DT or CB would help, but I'm not sure it will make them any more likely to get past the competition. There's no WR worth taking. They have 2 good tackles and a decent OL. By far their overwhelming need is a QB to build around. They could sign Alex Smith or trade for Matt Flynn, and both would probably be improvements, but is either going to be the long term answer? Flynn couldn't beat out Russell Wilson, so I don't think he's likely to be a franchise QB. Smith has proven he doesn't have the arm to scare anyone, though he's a decent game manager. The Browns could reach for someone like Matt Barkley at #6, but is it likely that he's a better prospect than Mallett, especially with 2 years sitting behind Brady and learning patiently? I personally doubt it.
The Browns don't have a 2nd round pick, courtesy of the supplemental draft. But if they traded #6 to NE for Mallett and our 1st they would essentially recoup their 2nd round pick (a few spots higher) and still get a QB to build around, at cheap cost for 2 years. They could get a solid WR, OL or defensive player at the end of the first round. It's not a bad way to go. I'm guessing that Mallett is more likely to be ready to start if given the opportunity than a rookie would be, though it's purely a guess. Lombardi probably has some idea about that, if from no one else than his son. It's a gamble, but it might be less risky with more of a chance of reward than any other option. Score, and you're golden. Miss, and you go back to the drawing board. What the Browns would have given up was taling Barrett Jones instead of Chance Warmack at OG, or taking Kawann Short instead of Star Lotulelei at DT, or taking Xavier Rhodes instead of Dee Milliner at CB, or something along those lines.