I'd order the extra UMC then if it makes you feel better about trips. I simply take my existing UMC with me when I'm going to be driving over 100 miles that day. You also have the J1772 adapter and I keep that in my car at all times. I am much more likely to find a public charger in town than a campground NEMA 14-50 outlet so that's kind of my hedge. You do need the UMC for 110V outlets though so keeping it in the car is an added layer of security.
Another option is to keep the UMC in the car, that's delivered with it, and build yourself a 75A OpenEVSE, cost for that is half of the HPWC ($600 versus $1200). There are three differences:
1. It takes some work, skill and effort to construct your own EVSE, you must be handy and have some tools.
2. You will need to use the J-1772 adapter with your own J-1772 EVSE, this leads to #3
3. You will need to press the "charge port open" before getting out of the car, as you won't have the little button on the handle to press to get the charge port door open
I asked Tesla about getting a spare J-1772 adapter, they quoted me $95, and added they are in very short supply, so it will be a wait to get one (the extra adapter is so one can be kept in the car, and one can be left on the J-1772 75A OpenEVSE
If you can live live with the listed issues and are handy, you can save about $500 ($600 for the 75A OpenEVSE construction + $95 for a spare adapter, so $700 total versus $1200)
For the curious, here is my 75A OpenEVSE wiki page:
http://code.google.com/p/open-evse/wiki/75AOpenEVSE