Except that it is extremely useful as a powerful calculator for students and professionals doing technical things, and in the process of being very handy teaches people mathematica notation. Just because it's not necessarily aimed at the average person doesn't mean its not aimed at anyone.
I expect Wolfram alpha has done a lot to drive sales of mathematica.
I feel the original comment applies not to mathematica, just to the (alpha - mathematica) subset, much of which seems to exist just to prove it can exist. But I don't begrudge them this at all, it's still cool.
Problem is, every time I think "hey this would be a problem WA could answer!", I somehow get stuck because WA never seems to have the data I'm looking for (last time, electricity prices are always assumed to be US prices even if I say "in Germany"), or it's unable to present me the data in a way that's helpful to me (last time, a bit longer ago, comparing some demographic statistics between 3 countries--I forgot the details).
There was one time it worked for me, when I had to compute the derivative of a Lennard-Jones potential function. Even though it gave me a lot of different answers of the same formula written in complex different ways. I just remembered Mathematica has a Simplify function, maybe that would have worked in WA too.
Entering one's birthday and finding out how many weeks or days you lived is kind of fun.
For general calculations and unit conversions I prefer to use Google Calculator, for the simple fact that the page loads much quicker.
Many students at my (non-US) university use Wolfram Alpha for graphing, simplification of formulas and similar every day, yet most have no idea that Mathematica even exists. If Wolfram wants Wolfram Alpha to drive sales of Mathematica they should do more to advertise that program through the service.
I expect Wolfram alpha has done a lot to drive sales of mathematica.