If you've got the time and money, yes.
If a diverse range of traffic is required, yes.
I know people who hate Google and refuse to use it, ever. There's still a market for other websites out there. Another thing to consider however is that fact that over time as general traffic increases, search engines other than Google should have no trouble finding you... at least the top few search engines anyway.
You may want to think about smaller PPC companies like 7 search, etc. They are a great way to test ads for very low cost per clicks. Once you have an Ad that is fine tuned you can move it over to one of the larger engines.