As much as I hate the Ott signing, he's better suited for 3rd/4th line minutes than Roy.
OK, I'll bite. Why? Roy is younger, more productive, cheaper and didn't get a multi-year deal.
There's a difference between a guy who's been with a team for 5 seasons and keeps turning the puck over because he's a puck hog and a guy who's been with the team 0 seasons and needs to adjust, etc. I am not excusing Ott's turnovers, I am saying your comparison is flawed in that a reasonable person could be ok with Ott given his short time with the Blues and not ok with Perron, who was supposed to grow into of the team's leaders and instead became a whimsical primadonna.
There 's also a difference between a guy who may well be a puck hog and turn the puck over a lot but also scores at a top-30-in-the-league clip
and generates a lot of shots and another guy who turns the puck over just as much (more actually now) and not only rarely scores but also takes lots of bad penalties.
Your top-30 claim for Perron is a bit rich as he only reached that sacred list once (and even then, almost, at 31st, in his first year as a run-and-gun Oiler). Scoring rankings of David Perron:
2008 - 09 season (81 GP): 15 goals (167th), 35 assists (77th), 50 points (108th), +16
2009 - 10 season (82 GP): 20 goals (103rd), 27 assists (139th), 47 points (120th), +13
ignoring 2010-11 as he only played 7 games due to injury
2011 - 12 season (52 GP, injury-shortened again): 21 goals, 21 assists, 42 points, -10. Projecting based on per-game stats, he could've been at 33/33/66/-16, which would rank him 17th in goals, 80th in assists and 33rd in points (with -16 +/- rating). And this would have been his best season because...
2012 - 13 season (48 GP, shortened due to lock-out): 10 goals (129th), 15 assists (144th), 25 points (139th), -16 (not a torrid pace for full season either).
2013 - 14 season (78 GP as an Oiler): 28 goals (31st - let's call this a top-30 place cause some guys tie), 29 assists (112th), 57 points (56th), -16.
So Perron reached the top-30 once. The only consistent thing about Perron is his +/- in the last two seasons. And while you would expect it from his season with the Oilers, the two season with negative +/- with the Blues while playing on their top line point to the huge liability that he was, despite the goals and assists (that weren't actually that good either).
Perron was maligned because he was playing on the top line and not producing nearly enough and not near his own potential, while being a defensive liability season after season. The expectations were high, and the disappointment was bitter. And he was given plenty of time and chances before fans started giving up on him. Ott is a different level signing. He fills the gap at 3rd-4th line center, so while Blues may have paid him too much (and looking at other, cheaper options, no thanks), but he can be effective in that place. And he deserves a little more time before we turn our backs on him (if that is to happen).