Tolkien was an author, not a filmmaker. But if you're arguing that this game has no dramatic flare, and therefore grand entryways have no place in it...
Never said that, just that DF tends to avoid ludicrous crap. I'd say that enormous doors probably count.
If DF was cinematic more than realistic, master wrestlers would be able to take on armies of goblins, single-handed. Try it some time.
But those are some of the most fun times with my wrestlers... Especially when they wear loincloths. It never ended well.
My point. DF tends to sacrifice cinematic physics for realism.
Dwarf fortress tries to be a more or less realistic game, set in a fantasy environement. So it takes clearly fantastical elements, and allows those to react by clearly defined rules of nature.
So, I say that we need to put this in, but players should not complain when their massive gate is easily smashed in by a giant, or requires more than 3 times the materials as a small one. All things should have good and bad sides. To illustrate it with a quote:
A game where there's one action that is clearly superior is not a game, but a puzzle
So let people build their gates, and let their dwarves admire them, and let them be brutally slaughtered when the goblins come.
But don't prioritize it over something players have a good reason to use.
Of course they should be vulnerable to siege beasts. Though building destroyers should be changed so that they demolish different constructions in different ways. After all, it's a bit unrealistic that they could destroy a fortified gate, but not the wall next to it. And you can completely stop them with a pile of naturally placed sand. Toady already plans to implement moving parts, and a portcullis is a moving part. But they only make sense if you build them in gateways. A portcullis, for those that don't know, is a lattice of wood or metal that would rise and descend in two vertical grooves cut into the walls of a castle entrance. They were usually built in pairs, with the outer one being up to let attackers in. That portcullis was then closed, trapping the enemy in a confined kill zone. Rocks and fire heated sand might then be dropped on them from the roof or through murder holes. Oil was never used, as it was far too expensive. Archers and crossbowmen would fire at the trapped attackers through arrow slits or the inner portcullis. That seems like a pretty good historical basis to expand upon.
Wait, hold on, there are people who don't know what portcullises are? Around here?
Aside from that, sounds reasonable, but portcullis =/= giant door.