Honestly I don't think the reason matters. Personally, I don't like NTR but I do like having a worthy adversary. So as long as I can beat him I do enjoy the challenge of it. I think most people who want NTR are going to lose to Eric on purpose though, which is also fine if that's what they like.
Where devs run into problems is when they make it so this rival is either guaranteed to win or guaranteed to lose. That's when people start getting irritated by it. In that scenario either the NTR fans or the vanilla fans are guaranteed to not get what they want.
I think most of the initial hate for Eric came from the fact that when he first appears he's an overpowered opponent unless you know exactly how the mechanics required to thwart him work (which is not likely in your first playthrough). The game doesn't really give you much info on how to beat him on your first playthrough until he's already gotten farther with Kira, Lisa, and Alice than non NTR fans would want. After you know how things work he's easy to thwart.
Where devs run into problems is when they make it so this rival is either guaranteed to win or guaranteed to lose. That's when people start getting irritated by it. In that scenario either the NTR fans or the vanilla fans are guaranteed to not get what they want.
I think most of the initial hate for Eric came from the fact that when he first appears he's an overpowered opponent unless you know exactly how the mechanics required to thwart him work (which is not likely in your first playthrough). The game doesn't really give you much info on how to beat him on your first playthrough until he's already gotten farther with Kira, Lisa, and Alice than non NTR fans would want. After you know how things work he's easy to thwart.