I think you should keep it in, instead of filtering it out. It will make the site feel more authentic, and the out-of-place quotes can become viral (in the sense of "no publicity is bad publicity").

I'd also humbly offer two enhancements: 1) Create widgets that other sites can place on their own web sites, linking back to yours. 2) New daily quotes could be published via RSS. (For example, I don't see myself visiting the site more than the initial one, but I can imagine someone putting an RSS feed into his daily reader to spice it up.)

Good luck.