When I was getting mine I went with 2 (one drain and one thermo). Quickly decided that I wanted a sightglass as well, so got one with a thermo included, so then I didn't need that port anymore. Now I use the 2nd port to whirlpool with the drain port, a pump, and my CFC, which has sped up cooling nicely. So I'd say a thermo port isn't required (especially if you're thinking about installing a sight glass yourself), but can come in handy in the future. Like was mentioned, you can always plug it for the time being if you aren't using it.
I didn't see you mention, how are you chilling? Ah, just saw you're also using this for heating mash water. In that case, or if you're using an IC, an installed thermo is pretty nice, since you don't have to keep standing up and leaning over to check on the temp you're at. You can just sit next to the pot with a beer and watch the thermo, shutting things down once its at the right spot. If you're draining hot wort from the pot and using a plate chiller or CFC, the installed thermo is less useful, but assuming the price point for the second port isn't outrageous, I'd still suggest getting it.
