"However, I believe that most rattlesnakes in the United States have hematoxic venom so a snake bite kit is a good idea a lot of times, in the US at least."
Yes, most poisonous snakes in the US are pit vipers, with the coral snake as one of the exceptions.
Here's a source for scientific literature on the actual effectiveness of the extractor, one of the ones that led WMI and other institutions to no longer recommend its use for snakebite:
The best recommendation is to clean and loosely dress the bite, keep the bitten limb at heart level if possible, and evacuate the patient immediately. For elapid bites (Coral snakes here and many others overseas), there's an elastic wrap technique that's gained some favor, but the advice is still to evacuate immediately, wrapped or not.
The one case where the extractor is still considered potentially effective is for insect bites and stings, though even there you run the risk of a bit of "collateral damage", and depending on the patient, administration of antihistamines or an epi pen could be the more immediate concern.
Ultimately, taking the extractor is a personal decision - learn as much as you can and don't take any advice, especially online, as gospel. Based on my training and research, I've chosen to go with the protocol and not take one, nor would I use it if it were at hand.