Not to forget... his superior sniping and I can only assume perfect optical vision!
Is just one out of A LOT of abilities he has!
No purely physical he isn't very strong in comparison to the others but... my god.. his enourmus skill set!
In many way he is the Straw Hat artist! he is by far the most creative crew member, not just in drawing skills but in every thing! his way of battling highly reflects this! Hell, I even think his lies/story telling he is able to instantly make up on the spot and tell is a great sign of his creative mind..
He is able to think in very creative ways and is able to immediatly adapt to the situation based on what he just happens to have around him, in an instant coming up with a new unique idea and put it into action.. instantly!
He is insanely good at actually making a battle stragedy and lure his enemies into traps, making them think he is up to something only for that to be the trap and something worse happened.
This is showcased multiple times, most noticeable with Usopp's battle with Luffy, where Usopp was owning that entire battle... until he ran out of traps.
But still, that says some-thing, that first when Usopp ran out of traps where Luffy able to beat him at all.
In the Perona battle Usopp did pretty much the same thing, tricking Perona into doing what he wanted, and then of course in the fishman island battle where he also build up his trap, forcing that fishman up in the air where he could move so Usopp could shoot him!
Usopp is a guy, genuinly able to beat people he shouldn't be able to beat, people much stronger than he is physically... with creativity, adaptability and traps.. he is able to beat people he really shouldn't have been able to beat.
So really, that he is strenght wise the weakest strawhat... doesn't matter a lot.
Because his true strenght doesn't even lie in his physical strenght, it lies in his mind and how he works!
In spite of being weak, Usopp is a scary ass dude, and I would NOT want to face him in battle! he would be sure to set up some kind of trap and get me from behind!
At least with Luffy you do see that fist coming right for you before it's over... With Usopp.. you never know where that finishing move will come from!