First, I think you miss the point. A bug is when something does not work as intended. If it works as intended, even if you don't like the way it was intended to work, it's not a bug.
Ahem. If someone's missing anything here, it's not me. Repeat. Bug. By. Design. Is. Still. A. Bug.
Second, how do you think your suggestion would improve the game? As I see it, it would only make base attacks easier, make base attacks more repetitive (always encountering them from the same place), and reduce the risks associated with a base attack.
Well, for one, it would make the base defense mission less idiotic.
Second, it would make it less frustrating. (I.e., you won't have to spend half an hour just to get to action).
Three, it would make it that easy only against dumb as doorknob lightly armed aliens. Against a wave of even 4-5, sniper towers would do little but attract fire.
Now, ask the same question about the current state. How does it *improve* the game? Well, drop the "how" from there first.
1) It doesn't make base attacks any more difficult (Right now, all you need is just stand guard behind a corner with a flamethrower, is all).
2) It makes base attacks even more repetitive (you still do it from the same place, the entrance, the only difference is just *getting there*).
If you want a suggestion how to improve it... Unlock the access points. As of now, aliens can get in only via the large hangar (one-way) and the entrance. There are at least three other building types which [should] have surface entrances, even if locked.
And repeat: as of now, base attacks are as repetitive as they could be. With legwork attached... It's THE most tedious (not hard) mission. (Second place is Big City, for the similar reason).
Again, tedious != hard nor difficult.