Very.
Although, I must say, I don't get it as much for the Lakers (or Hornets for that matter). I can see the Celtics, even Suns (if they weren't so cheap) or maybe Pistons giving Posey 4 years at the full MLE. For those teams, they have a closing window and Posey is a guy who can help a team A LOT in the next two seasons. If he helps put you over the top (the way he did with the Celtics this year), it's worth it for those teams looking for one last run, even though you know he'll be a much lesser player in the third, fourth and maybe fifth seasons of the contract. Posey's a guy who's probably worth the full MLE this year, and maybe next, but in his third year, you'll be thinking he makes twice as much as he deserves, and in the fourth year it'll probably be worse.
For the Celtics, it's doable because the team is led by stars in their 30s. They have to do what they can to win as much as they can in the next 4 seasons. And during that time, our big contracts will gradually be decreasing so overpaying for one guy won't be quite so bad (I can see Ray, Paul and KG all signing for less when their contracts expire in 2, 3 and 4 seasons) because the stars are probably going to give us more financial freedom. But for the Lakers, they're looking at probably 6-8 more max salary years for Kobe, big money for Gasol for the next 7-8 years and big money/max contracts for Bynum for the next decade. They aren't going to have as much to spend as the Celtics in 3 years' time, which is when Posey's going to start looking like a bad signing. So overpaying a guy like Posey becomes worse and worse as time goes on for a young team like the Lakers (or Hornets) because it ties them up later on when they're still contending. For the Celtics, we're expecting rebuilding in 4-5 years, so why not keep going all in right now?