I absolutely agree with your first statemen and definitely I can see where you are coming from. Sure, some people's "play style" is to remove as much challenge from the game as possible (i.e. underground and minibike horde nights). Those are the only two play style options you mentioned and I'm fine with people using them because they are logical choices in reality... I would never team up with someone with that play style though because I don't go that route. I chose not to play that way because I like the challenge and risk.
There are more options than just tower defense, underground, and minibike cruising though. I like to mix it up a bit. I do enjoy building a base and setting defences and trying new things I did not try in my previous builds to still get that trial and error feeling back from my early hours of playing. But I also like to get out and explore... Not just to scavange, but survive in different biomes and get caught in random situations. My most played map no doubt has more than 15 different mini bases/storage areas accross the map. To me, that is a different play style outside of tower defence. Sure, a lot of horde nights I find myself back at one of my bases I've ramped up specifically for horde nights, but sometimes I'm out in the open fighting off the horde with what I have on me. Mixing up play styles is what I do to keep the game exciting.
So, the point I was making in my original post is tower defence is a main staple in the game, but not a requirement. There are other styles that can be used to mix it up without losing the danger... in fact, increasing it.
You are also correct, a stealthy play style doesn't really work well in this game as of today but maybe that will change here soon...? It would be awesome if the skill system gave this category a little love. I would love to be able to throw a rock or bullet casing to attract zombies away so I can get into an area undetected... Or drop some fresh meat next to a barrel of gas and wait for the zombies to gather around before I shoot the barrel. Wouldn't be my main play style, but I would add things like this to my bag of tricks for sure.