I've never heard of this... did you hear this from a rep?
People do this all of the time. I wouldn't bother asking a rep, half the time they don't know what they are talking about.
You can turn the MHS feature on and off by logging online or using the MyVerizon app on your phone. You only get charged for each day it is on.
Unlike plan changes (which are limited to a certain number per billing cycle), you can turn the MHS off and on as much as you want in a billing cycle. You only get charged for what you use.
Now, you may see a charge for the full month for the next month, but you get that credited back. It can make your bill look a mess, but if you add it up, you only get charged for what you use.
Don't you have to pay for hotspot if you're unlimited?
Like the OP said, it is $30/mo for the MHS feature, so $1/day. That is true for the legacy Nationwide plans regardless of whether or not you have UDP.
If you jailbreak and pay for a tethering app, it is free to use the MHS feature.