So to dispel some information here:
First, the Thunderbolt (and once its released, the Charge) have unlimited, free hotspot use until May 15th. Once that date comes, we will be releasing new mobile tethering/hotspot plans for 4G devices. The current $20/2GB MBBC/MHS plan is for 3G devices only, and most likely will never reach a 4G phone.
Now, just to be clear (or unclear, actually), there was a rather vague statement we saw that 4G MBBC/MHS plans will always tap into your normal data bucket (in other words, if you have, for example, a $15/150MB data plan, that can be for general phone use AND hotspot). I logically think this was just miswritten, but its possible.
Secondly, VZW has no currently policies that I have seen to charge overages to someone that uses excessive amounts of normal phone data if they have an unlimited data plan. I did seen a PSA that top brass at VZW did announce something about throttling the top 5%, but I have talked in depth with users that topped well over 150GB of data usage on 3/4G and never noticed a decrease in speed. Maybe there are people that use well over that, maybe that policy has not gone into effect... I do not know.
I do not know what data plans will look like later this year... will the $29.99/unlimited plan stay? Will MBBC/MHS plans get cheaper or more expensive? What I do know is that I bought a Thunderbolt despite knowing the battery life may be poor because I like HTC *and* because I want to make sure I have my unlimited data plan on 4G just in case things go south. That's just me speaking on a personal level.
Now, speaking outside of my professional standpoint, I personally am not sure (others may be able to answer this) if root-access wireless tethering supports LTE yet. It would make sense that it does, but I have not done it personally.
A temporary root is like being granted temporary administrator access on your Windows PC. It last until you do a factory data reset. I have seen with temp roots that custom ROMs/kernels tend to not work as well. A permanent root can only be undone by unrooting the phone. A factory data reset will NOT remove a permanent root. Be wary of rooting if you intend to return in a hurry. We do check to see if the software, firmware, baseband, ROM, kernel, and system file has been tampered with. If that happens, VZW will charge you the full retail of the device, which is like $570. If you intend to return it, do not mess with rooting if you are unsure. I only recommend rooting the device if the device is stable by itself (in other words, you will not have to return it), and you have no intentions on returning it. You bought it, you own it, do what you want.