My theory is (and it's just a theory) that unless you're using a TON of data, they're never going to check anyway.
I've somehow managed to upgrade my phone twice, and use hotspot from time to time too, and for whatever reason still have unlimited data on my plan...but I don't even care that much because I rarely use over 2GB as it is...and maybe that's why Verizon doesn't care either?
EDIT: I guess my point is that if keeping unlimited is very important to you and you plan to tether quite a bit, it may be worth it to you to pay for the MH service...but if you're like me and just plan to use it in small doses from time to time when you need it, then you're probably pretty safe. I emphasize "probably"...obviously anyone doing this understands the risk involved when it comes to Verizon's policy on tethering with an unlimited plan.