With two people you save about US$20 from municipal, each way, after factoring in the cab fare. With three people, you save around Us$40. With four people, around US$60. Whether it's worth it depends on how tight you are with a buck.
Taking the water taxi, with two, you save around US$45 over the international; with three, US$75; with four around US$110.
For me, with two people, no. With three or four, yes.
As someone else said, it also gives you a little better perspective on Belize. Too many folks come to Belize and never see anything except San Pedro and the international airport.