Without getting into the contract and so on (it is a factor, of course), here's why the Toronto Maple Leafs would consider Roberto Luongo: Over the last three seasons, Luongo's save percentage is .920.
By inserting that into Toronto's lineup in 2011-2012 for 60 games, with the Leafs allowing an average of 30.8 shots against (same as Vancouver, incidentally), then Toronto, with no change in performance (ie. 900 SV%) from James Reimer in the remaining games, would allow 2.55 goals against per game, down from 3.16 per game in 2011-2012.
That difference of .61 goals per game, over 82 games, amounts to a 50-goal difference. Adding 50 goals onto Toronto's 2011-2012 goal differential would have the Leafs at a plus-17, which would be better than six of this year's playoff teams.
Now, Luongo is going to be 33, so his play may start to decline in the not-too-distant future (maybe next season, maybe in three years, maybe in five) and that .920 save percentage won't be achieved, but to suggest, as some have, that Luongo couldn't get the Maple Leafs into the playoffs misses just how big the difference is between Luongo, an above-average NHL starting goaltender, and what the Maple Leafs were getting from their netminders this season.
Justifying the financial commitment? That's another matter.