I am both licensed in the state of Mississippi and currently marketing for wholesale leads.
Pros: The MLS access obviously grants you the convenience of reliable comps that people without it do not have. You can also potentially get listings from leads that don't fit the wholesale model. Like many people say on BP, it's just another tool to have on your investor belt.
Cons: I can speak from experience on this one. You will certainly not make any realtor friends mailing to their listings. I've gotten several angry agent calls inquiring about why I'm "marketing to their property." Even though my letters clearly state both that I am licensed and intend to purchase the property and not list it, the calls still come and they are not pleasant to deal with. Just today I received two. One realtor emailed my broker and informed her that what I'm doing is immoral and misleading. The other realtor called me and asked why I sent a letter to her listing saying that I wanted to buy a 7 million dollar property in all cash and within 30 days. In the last two months of marketing for wholesale leads, I've made about half a dozen realtors not like me at the very least. So unless you are prepared to go through all of the hundreds/thousands of your addresses to make sure that they aren't on the MLS first, you're absolutely going to mail to houses that are currently listed with another realtor and odds are they aren't going to like it. I haven't figured out how to address this problem yet so if anyone has experience dealing with this please let me know!
Good luck!