Well, I can't speak for everyone here, but I know why I am interested in dropping a power adder in a V6 instead of buying a V8. One reason only. When I walked in to buy my car, the V6 was on the showroom floor and a V8 was not. I didn't wait 3 months to get my car, I drove it home in 2 hours. If they had a convertible V8 in stock, I probably would have bought it. It's not about money, it's not about insurance, it's not about economy. It was simply a matter of what was in stock and what was not.
Now don't misunderstand. I do not in any way wish I had purchased a GT. I do not regret my purchase. You see, when I was a kid, I wanted terribly bad one of those 86-87 convertible mustangs. They were brand new back then and it was the coolest car on the road in my opinion. I never cared for the body style they went to after that, but in 2005, I fell back in love. I was going to purchase a convertible mustang and that was the end of it. Well I did, and it's a V6.
Now that I have it, I want to get the most out of it. Part of that is putting on a power adder. Most likely, in my case, a supercharger. I'm not trying to turn my car into a V8. I'm not trying to blow the doors of a GT. I simply want it to be faster than it once was. My goal is to increase performance of the stock setup. My goal would have been the same if there had been a GT on the lot when I bought my car. I would have dropped a supercharger in it as well. Again, not to compete with anything other than what it was.
I don’t intend to race GTs. I don’t intend to race other V6s. I don’t street race at all and have no intention of going to the drag strip. For me, this is the first car I ever got that I really wanted. In fact, it is the first car I bother to buy new. Now that I have it, I want it to be the best that it can be.
After all that, I suppose the answer to your question is this:
I bought a V6 because it was there, and I want to make it faster because I can.