You'd like to have all-pros at every position especially at the key positions like LT. Leno is not in that class but he's a very solid guy.
I wish I had the PFF stats to see how he grades out in comparison to other lineman, but I don't. I do know he only gave up 4.5 sacks in 16 games he started and had 6 penalties. Not bad. And the Bears O-line as a whole pass protected very well last season and we know they run blocked as evident from Howard's 1300 plus yards.
If you don't re-sign him he could become a FA after this season, which means you may have to dedicate resources to find another LT, which isn't easy.
Now if the planets align and there's a blue chip LT available when we draft AND he is head and shoulders better than any available corner, safety, OLB, WR, or DE then by all means draft him but signing Leno gives you some flexibility to draft/sign other positions of need.
Next up - Akeem Hicks.