You are correct; you have a valid point. But keep this in mind; Windows Phone users think differently than iOS, Android, and even Blackberry users. If a user of any of the other three platforms wish to buy a phone, it doesn't matter which carrier they use. They can either buy one of the factory unlocked phones that work on Verizon, Sprint, and US Cellular, or they can buy a carrier version of the phone they want. Windows Phone users are unique in that they do not have this option. They need to choose the phone first, and then the carrier.
Since Windows Phone users are the only ones where the carrier determines which phones are available, and they make up about 1% of smartphone users, this matters to almost nobody.
So, if 99% of the smartphone users can choose whichever carrier they want and still get the phone they want, do you think they will choose a carrier based on whether or not a WP is available? They will look at things like, does it work where I need live and work? They odds are higher with Verizon than with AT&T.
Another thing to remember is that this is only counting LTE coverage. Verizon's voice and SMS are using CDMA, which has better range. This adds even more area where you will get at least voice coverage. Verizon's advantage here is likely even higher than the 33% more LTE coverage.
Again, I am not discrediting at all the fact that it doesn't matter to me if AT&T doesn't have coverage in a remote town in Wyoming with a population of 12. It doesn't. I live in a heavily populated rural area where I have acceptable coverage with any of the four major carriers. But when I head out west, which I do fairly often, Verizon is the only carrier that gives me consistent coverage everywhere.
Many people are in a similar situation. Or, even if they aren't, this is where the marketing ploy you mentioned comes into play. All else being equal, why would they choose a carrier with 15%, 40%, or 60% less coverage? They're going to walk into a carrier store and buy a branded and locked phone anyway. Whether or not the carrier uses GSM or CDMA* makes absolutely no difference. In fact, Verizon's phones are better than the other carriers in that they are SIM unlocked. You can take a Verizon phone to AT&T or T-Mobile or any other GSM carrier, but you cannot do it the other way around with jumping through the hoops.