I couldn't agree more that building as much as possible in the traditional manner is the very lifeblood of mapping with Duke.
I personally think that models are best used to enhance the gameplay you create with your levels. I love to be able to walk along through a Mapster-built, Polymer lighting-enhanced, uniquely-textured level, and occasionally take a peek out a window or over a fence to see a glorious model doing it's thing in whatever manner was intended. It's almost like breaking a 4th wall... Only not. It's hard to explain. I have one level where, as you're moving through the stage along the ground through some areas thick with trees in the middle of the night, you get a brief moment to stop and look through a gap over a cliff... What you see is a path heading down a mountainside in front of you, out into the distance to a brilliantly realised, detailed city far off, complete with all sorts of buildings, and all the little lights of the vehicles and people moving about amongst it all. It's something that I simply couldn't have achieved to such effect without the support of models, at least not in any way as atmospherically as I managed... But the best part is it just blends seamlessly into the whole 'mythos' of the environment - I'm not sitting there going "Wow, this MODEL looks great!", I'm just enjoying the sight.
Achieving that perfect fusion between hand crafted levels and modelled enhancements is critical, in my opinion. Sprites and sectors alone can only get you so far these days, especially when what you dream to achieve far outstrips what the stock engine can provide. I hate the idea of crafting almost everything out of models though, that would essentially defeat the purpose of using this engine in the first place. But using such things to merely improve the work you've already done is fantastic - especially when used with a little creativity and not just slapped in left right & centre, for shits and giggles.
Creativity is key, and while it may not be apparent (at least from me), I sure as hell like to try my best.
God damn I rant on a bit sometimes. It's just that I never feel like I've made my point convincingly enough