I just said "Blocks Sleep Powder and Hail." also, not just fire attacks can OHKO forretress.lol @ "only". Immunity to Sleep Powder and residual damage from hail far encompasses the situational utility provided by Sturdy. The only time Sturdy would be of use is if you plan to sack Forretress to check something or if it needs to take a hit from a random fire move to set up Stealth Rock, both of which are rather questionable uses for it. Forretress will be switching into entry hazards to spin them away -- ergo, Sturdy is nullified and inconsequential in almost every situation. To be fair, both abilities don't necessarily have application in most scenarios, but at least Overcoat can find some actual use, whereas Sturdy conflicts almost entirely with Forretress's primary role.
I'm just saying that you can use other pokemon for Overcoat such as Mandibuzz, or to block sleep powder/spore by Sap Sipper Ability. Overcoat kinda doesn't suit Forretress that good enough than Sturdy. :)
Sturdy Stealth Rock's also not just the point at using Sturdy. You can use Sturdy Red Card to ward off Boosted Sweepers.