I know, and that's a fair point, but there's a difference between a place that's naturally going to have kids, like a high school or a small town, whether or not it's atop a Hellmouth or whatever tech they teched at Coal Hill Academy, and deep space.
Planets are not guaranteed to be safe. There are earthquakes and tsunamis and hurricanes and plagues and asteroid impacts and who knows what else. Not to mention how many people die every day just driving to work. At least on a starship you're surrounded by top-of-the-line defenses and trained defenders. As long as it's a vessel whose primary mission is exploration rather than actively seeking combat, I don't accept the assumption that it's intrinsically more dangerous than everyday life.
I know that was the intention, but I think it's kind of a weak rationalization. If there's a battle, why would you want to lose some of your armaments and half your power supply?
But it's not. The stardrive/battle section was specifically
designed to be an independent combat vessel in its own right, with all the power and weapons it needed to do that job. It just happened to be mated to a saucer-shaped research station that it carried around on its neck and could leave behind when it needed to go into battle. The idea was that the battle section would be more effective in combat
without the saucer, because it would be lighter and more maneuverable and able to devote all its power to battle instead of needing to handle life support, shielding, etc. for that massive habitat section. Consider how compact the
Defiant is in DS9. Smaller can be better for a battleship.
This whole idea that the ship is more effective in combat with both hulls is nothing but a retcon after the fact to justify the visual-effects producers' desire to stop working with the separable model. It goes against the original intent of the design. After all, the battle section has the warp engines, which are immensely more powerful than the saucer's impulse engines. Having the same warp engine power with less mass to move around hardly seems like a loss to me.
Also, the saucer can't warp away from danger and it was relatively lightly armed if it did have to defend itself.
On the contrary, it was heavily armed specifically for the purpose of defending the crew. The saucer had wraparound phaser strips both above and below and an aft-firing torpedo tube in the undercut (since it would mainly need to fire on a pursuer while retreating). In fact, the saucer had considerably more phaser coverage than the battle section did. Again, this is exactly how it was designed to work, so of course all of this was considered in the design.
Also, I think the idea was supposed to be that if the ship needed to go into battle, they would leave the saucer section and the civilians behind someplace safe
before going to where the battle was. That's pretty much what Geordi did in "The Arsenal of Freedom." It's just that the separation maneuver was used so rarely that the real intention didn't really have a chance to come across.