all of our paysites have a phone billing option (thats the new term for dialer, since most of the time people using them are on broadband), we use GXBill and it was the easiest thing that we ever did to add an extra 10% to our bottom line.

of course, if you are talking about the old fashioned downloadable exe dialer, those are totally gone for two reasons:
1) anti-spyware / anti-virus programs all block them
2) most surfers are on broadband and don't have computers hooked up to phone lines (hell, a lot of computers don't even have a modem in them anymore)