One of my closest friends carefully works out optimal strategies for his list, and develops several tactical options each turn to account for what his opponent might do. So he definitely wants to win and is good at it.
Most of my army lists are developed on strict fluff basis; until Obliterators were given the possibility of having the Mark of Tzeentch I did not even consider having them in my army. While I have no objection to winning, most of the time I would rather lose with a fluffy strategy than win with a more competitive one.
However, all of our games are enjoyable.
Because we are both pleasant people. I do not get angry because his list does not have rigorous fluff credentials. He does not get angry because I have made it harder for me to win.
Some of you will probably be thinking that this is all right for casual games but you want a real challenge. In which case maybe you should be playing more games against casual gamers. If we look at the real world, professionals in many competitive fields say the hardest opponent is an amateur because you cannot predict what they are going to do.
Looking at it another way, where do you want your challenge: on the table or over the top of it? Which would you actually prefer, a game where it was slightly easier to win, or a game where you won but you had to keep stopping to argue about rules, LoS, and a thousand-and-one other niggles?
Asking whether someone is looking for the best list possible or a more thematic list will always have its place; especially as there are so many different ideas about what is properly fluffy. But maybe we should be dividing ourselves not into casual and competitive, but rather into gentlepersons and ruffians (or whatever the youth of today call each other).
Do you have a different view? Do you need more challenge than an opponent who is not trying to lose? Do you need an opponent to have more rationale for a list than not technically breaking the rules?