If you want to pre-arrange a taxi, you could contact TTOL sponsor Wilfred Brooks (check for his info on the sponsors list). As far as evenings in Grand Case, I believe the guy is still around that has a golf cart type situation that you can get rides for something like $10 roundtrip to the restaurants within Grand Case itself. Talk to your hotel front desk about that.

As far as rental car in general, especially this time of year, it's going to be cheaper to rent a car, versus paying for trips to Anse Marcel and Orient Beach, plus your transfers to and from the airport, which will probably run you about $35 each way, including bags to go to and from the airport. As far as missing out on things, it definitely takes a lot of spontaneity out of the trip, if any time you want to go somewhere, you have to track down a taxi. Can't imagine doing that myself on SXM, but everyone is different.