HNMs should have longer spawns, its part of what makes them different then the plain old NMs. They are much harder, have longer spawn times and drop some of the better gear at slower rates (lower drop rates basically). I would actually be pretty bored if an HNM poped every hour and the drop rate is good...then everyone an their grandmothers would have good stuff. A major part of the HNM appeal was how the items were much rarer to get, making the people who got them slightly above others. As much as people dont like to hear it, competitiveness and showing off is a huge appeal in MMOs.

I can't tell you how many times i ran by and checked people in FFXI and went "whoa...i want that" and knew right away that the person who got it was not only lucky, but put a lot of effort into getting the item, its a big feeling of accomplishment and excitement when you get that hard to get rare drop for yourself or friends.