Pros DO make a difference. They made a difference to me when I was newer and aspired to be a better disc golfer....when I was buying and trying a whole lot of discs! Most of the time based on what is popular/"successful", based on what's said on this forum, or because of the pros themselves. Remember, they're the best players in the world and that means something to somebody! Pros even still make some difference to me because I love to watch them! Although I buy and try extremely few discs and I don't care what they throw anymore outside of my ever morbid interest.
I personally can't quantify what difference some pro might make to a company in net income, but it appears to me that it makes a difference. Is that difference sometimes a "big deal", I speculate yes. Is it usually a "big deal", I speculate no. Of course it is dependent on the year, the company, the pro, etc.
It is not apparent to me why MVP doesn't bother to sponsor pros. Obviously it probably has something to do with the company not wanting to or not caring about sponsoring pros, not thinking it is worth the investment, or possibly they have tried and failed so far. Frankly they don't have a destroyer+ speed discs yet, if they ever can or will make any, and if that meant their pros would require an open bag if only for that slot, then it would be awkward.