Let's be honest here, the problem really isn't the injury history. Take a look at a guy like Malkin. He is injured every single year and rarely makes it over 70 games, and no one really asks him to block the same amount of shots as Tanev, yet any trade involving Malkin will get full value. Now clearly this is not a good comparison because Malkin is arguably the 3rd best player in hockey, but it does paint the picture for me. The problem with Tanev is not his injury history, it's his pedigree and the fact that he puts up no points, among other smaller things. Had he been drafted in one of the first two rounds and had he put up more than his projected 22 points a season (so basically if he were more like Vlasic), then clearly public opinion would be much higher as he'd also have had much more exposure.
Given a fan base as big as Toronto's, where the majority of fans that follow the team are generally uninformed about other teams, it's hard for management to justify giving up a couple of decently valuable assets for a defenseman who doesn't put up points, who hasn't really got much of a personality, who isn't flashy, in fact barely noticeable (in a good sense), who isn't very physical either. Tanev is that one player on every team who does all the small things right, but only people with a background in hockey really notice it. There's huge value in that for any team and it would clearly help the Leafs contend in the playoffs, but there is an image issue. Also risk. Toronto just got rid of Lupul in a rather weird way that generated a bit of media buzz. If the same thing were to happen to Tanev, it really could lead to reprecussions. And since Toronto hasn't won a cup in who knows how long... and now is their best chance, they might want to stay away from non-calculable risks. With this I mean that no one can really tell how the next season is going to go for Tanev. He could rebound to the form he showed at the beginning of the year and put up a 40 point season, or he could get a career-ending injury.
Tanev is not a good fit for Toronto, but not because of the player. It's because of the circumstances. Vancouver is better off keeping him for Quinn. This trade deadline we should have enough chips in possibly Edler, MDZ and Gagner which we could use to trade for a couple of picks. But we all know Benning won't trade anyone anyway. He's made something like 7 trades in the past 3 years and he didn't do very well on half of them. Either way, Toronto really isn't a good fit.