There is definitely potential if you can find a close spelling to a search term that gets thousands of searches a day. A few fat-fingered typists are bound to stumble upon your site. If you find enough of these you can get a nice steady stream of traffic coming. Unfortunately, you have to look a bit dumb yourself in order to capture that traffic, but whatever works...

I never thought of it that way, but you have a point. It does JUMP out of an email when I see them. My emails manage to get through spam filters w/o using them, so I've never given it much thought before..

It can also make you look like you have little education; perhaps worse - poor attention to detail. I should have paid more attention in High School English Class, but like Homer Simpson Says: "What do I need to learn English for? - I'm never going to England". How's that for some butchered grammar?

Depends on your market. You speak/write at the level of your intended audience. I make enough errors on my own so I try to correct them when I see them.