In combat you basically have to hit to actually do anything. (Maybe with the exception of the tactical warlord who can have other characters do the attack).
If you take a basic attack doing 1d8+mod damage with a longsword vs an AC 17 opponent you have the following average damage after accounting for to-hit chance:
Stat 16 - to hit-chance = 17-3-3 = 50% and 4.5+3 = 7.5 dmg = 3.75 dmg
Stat 18 - to hit-chance = 17-4-3 = 55% and 4.5+4 = 8.5 dmg = 4.67 dmg
Stat 20 - to hit-chance = 17-5-3 = 60% and 4.5+5 = 9.5 dmg = 5.70 dmg
Stat 20 vs Stat 16 = 52% more damage
Stat 18 vs Stat 16 = 24% more damage
That is A LOT at lower levels! The difference is going to be lower at higher levels. If you can get powers that do something on a miss, it matters even less.
(AND a Stat 20 vs Stat 18 = 22% more damage, still a lot if you ask me)
I would NOT play a character starting with a 16. Those extra misses and the low damage would be a huge turn-off. Missing with my encounter/dailies would be frustrating.
Playing characters with an 18 is quite acceptable, maybe even a 17 starting if it's at level 4 or higher. A tactical warlord might be an exception to this general rule, but I think I would get annoyed even then.
