SMS works fine on the 8700, but some people rather leave voicemail

The person on the other end may not be using a BlackBerry, and therefore, easier for them to get a voicemail -- since they may not be familiar with the procedure of reading a SMS.

Although SMS (aka "texting") is now getting more common in North America nowadays, you still have to verify that the other party is familiar with it in North America -- unlike Europe where it is assumed that everyone with a cellphone knows how to read received text messages.