Eventually, everyone will have lots more at-wills to choose from. But, bear with me for a second here...
Quantify how much power an at-will adds to the _most powerful use of a third at will_ for, say, a 15th level character.
As a guesstimate, the best case is probably the equivalent of 'able to catch one extra opponent with an attack, each encounter', which is, say, 1d8+6+4+2 damage, or 16.5. Taking weapon focus adds 2 damage to every single attack, which is probably at least 18 damage.
In many, many cases, the extra at-will will not actually be helpful, at all. Even for classes that would like another at-will, like the Rogue, say, it doesn't mean that the advantage of the extra at-will will even come up... you're using all of your encounter abilities, maybe one daily, and then your four or so at-wills. Maybe your third at-will option will give you the equivalent of another +2 damage or +1 move. Maybe.
It's peace of mind. It's happy variety. Breaking things? No. It does let people who are insecure with only two at-wills have _an_ option other than human, though, yes.