I mostly agree, but I also think there's a worthwhile distinction here between "need" and "want." I doubt anyone wishes for it to be the case that forts consistently perish from unmet "needs" without trade, but if the caravans could offer something that you wanted badly enough, that could still outweigh the risk or annoyance of clearing a path for them.
Aside from enough Gold to literally pave your streets in it, just to prove you could, what does a DF player WANT that he doesn't NEED? Aside, maybe from water, magma, and maybe more logs, all of which are available currently in theoretically infinite quantities.
I've recently made
a thread on the subject, but currently, dwarves only really need food, drink, shelter, some nice statues in the dining hall, and nothing actively killing them, and they are set for life.
I guess it depends which dwarves you're talking about. Dwarven footsoldiers are hardly rare in fantasy settings (whereas timid cowardice and heavy reliance on elaborate gadgets are more often characteristics of gnomes). I figure that Toady's dwarves are whatever he decides they need to be to make DF work, so if he designs the game to require a military, then a military thereby becomes dwarvenly.
Doing things in a simple, straightforward manner, when there is magma to be funneled and diverted straight from the very bowels of the earth?! HERESY! Magma (possibly when mixed with water) shall always be the DWARVEN answer to any military problem.