The funny thing about this is that the one edition this was NOT true in was 3rd edition.
As Nifft argues, you could gain attack bonuses from a lot of sources in 3rd edition that enabled you to make up for the attack deficit vis a vis the 18 starting stat guy. (A few that Nifft forgot to mention--stacking attack bonuses and larger attack bonuses. For instance, 3rd edition charging is +2 not +1 and a prone, stunned, flanked, and tanglefoot bagged foe nets you +8 to hit with a -2 to the armor class of the foe (maybe +11 if you manage to jump onto a table as a part of your charge to gain +2 for charging and +1 for higher ground); in 4th edition, there is no tanglefoot bag and your bonus to hit a stunned, prone, and flanked foe is still +2 for non-stacking combat advantage). In addition, the math worked differently as melee and ranged attacks at least were expected to hit most of the time as you increased in level. The difference between miss on a 1 and miss on a 1 is not that significant. Now, it's not always a miss on a 1 situation in 3.x but for characters who are focused on physical combat, it often is. And even when it's not, 80% and 85% are both pretty likely to hit. On the other hand, when we're playing in the 40-60% range, you have to scrabble for every 5% increment you can get.
You also could much more easily take advantage of multiclassing to benefit from different stats. And, perhaps most importantly, it was quite possible to make an effective character who would only very rarely roll an attack. If he cleric who casts recitation in the first round of combat, cure moderate wounds in the second, and searing light in the third round of combat doesn't have a good DC on his spells, who cares? Nothing he does is affected by his primary stat (except spells per day). Likewise, a wizard could happily make do with magic missile, haste, displacement, solid fog, and freezing fog, or a druid could stick to summon nature's ally spells. There are a lot of character classes and concepts in 3rd edition that work just fine with a starting 15 in the primary stat and don't improve too much by increasing that. From what I can tell thus far, 4th edition (by design) does not have similar options. Your character should be rolling an attack roll every round.
mach1.9pants said:
If you are a min/maxer yes you should have 18+ in your primary ability score.
Same as every edition.
But 4E is the first edition that guarantees an 18 as the standard.