Why would any interstellar starship still bother with streamline body design?
Let's not debate whether is there any friction while traveling inside vacuum space and assume all ships can brave a perfect storm brewing inside the giant molecular cloud. OK let's get down to business and tell me why my generation ship turns into a tuna? I remembered asking for a tetrahedron to save trillions of dollars so why should I pay that much to cross the galaxy? I see it also comes with sail and antimatter reactors. (No FTL upgrade option)
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It's a combination of several factors, most of which have already been mentioned but I'll put them all together anyway, with some helpful analogies.
1. Aesthetics/Rule of Cool
For this I'll use the analogy of a skyscraper. The most efficient design for a skyscraper, in terms of using all the available space, is a big tall cuboid. But you won't find many modern skyscrapers that look like that because big tall cuboids are really boring to look at. The Burj al-Arab, for example, looks like a giant sail. Freedom Tower tapers neatly along its edges. The Gherkin looks like a... gherkin. Alternately, look at cars. What looks cooler, a boxy, bulky Land Rover, or a smooth, streamlined Aston Martin? Actually, on the subject of sports cars...
2. Drag Reduction
(got ninja'd by @tom on this one)
You're right that friction isn't really a concern in space, but I'm assuming that at some point, your interstellar ship will want to land on a planet and then take off from it again. And when you're leaving or entering a planetary atmosphere - especially when you're entering - friction is a very big concern. Friction is (as far as I'm aware) what generates the massive temperatures that cause objects to burn up in our atmosphere, so unless you want your ship to disintegrate upon re-entry, streamlining - and decent heat protection - are very important.
3. Deflecting Foreign Objects
As @nzaman and @Burki have already pointed out, an interstellar ship will be travelling very, very fast. Fast enough that hitting something even the size of a pebble could cause major damage due to the relative velocity. IIRC from Wikipedia (I'll look it up later when I have time), a 6g piece of metal hitting your ship at orbital velocity will leave a 3-inch crater... and your ship will be travelling a heck of a lot faster than orbital velocity. Also, going back to point #2... space junk. If we have it, other advanced planets probably have it as well, to a greater or lesser degree, and some of it will be too small to detect and dodge.
A streamlined design will have two benefits here. First, it'll reduce your ship's frontal cross-section and thus reduce the chance of that evil space pebble hitting your ship in the first place. Second, you know how tanks have sloped armour? That's to increase the relative thickness: an object hitting a metal plate at a 45-degree angle has a lot more material to go through than if it hits it straight on. So it'll also reduce the damage caused by those tiny pieces of space debris, and increase the chance of them just deflecting off.
4. Saving Weight/Material
Interstellar spaceships are, as you hinted in the OP, expensive. Really expensive. And the materials required to build them might be hard to come by. So you'll want to cut costs. That means that you'll want your ship to have the smallest surface area possible without compromising on the interior space, not just for aerodynamic purposes, but so you can use the absolute minimum amount of material necessary. This will also save weight, which depending on your propulsion system, might make your ship more fuel-efficient and save you even more money in the long run.
So yes, there are plenty of reasons you'd want (or even need) your interstellar ship to be streamlined.
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