A "science ship" that's armed to the teeth
For defense. A battleship is not just a ship that's capable of battle, it's a ship that's intended specifically for battle above or to the exclusion of all other functions. The Enterprise's combat capability was intended to be a last resort when all else failed. Being able to defend yourself is not the same thing as actively picking fights.
and is regularly send into battle or to project strength to stop other species from trying something stupid.
Which, again, was an erosion of the original intent. I'm talking about why the creators of TNG chose to depict it as a ship with civilians and families aboard, and gave it the ability to separate in their defense. It's unfortunate that later creators neglected that intention and portrayed it in a way that made the presence of civilians harder to justify.
Plus, there might be times when separating and having multiple phasers from numerous different angles might be a bertter choice than all at a similar angle.
That's still operating from the assumption that the ship was intended for battle as a regular thing. Combat was something to be avoided if at all possible, not something the ship was specifically made to optimize.
I forgot about that. "Farpoint" demonstrated that in plain sight as well. For known attacks, they'd have plenty of time to pop the top then move to battle. But the only problem with that is the possibility of surprise attack. Which was thankfully few during the course of the run of the show.
There's a risk of surprise attack on anything, such as a colony town or a starbase.
In the event of a surprise attack, the priority would be to get away and keep the crew safe. If necessary, as in "Arsenal," the battle hull could then separate and return to deal with the threat.
I recall reading somewhere that the makers had trouble finding ways to do saucer separations while keeping the dramatic flow and story from screeching to a halt. And one can't do "Arsenal" or TBOBW every week either. They could have done it a little more often, but on a scripting level it IS hard to put in some of these nuances without grinding things to a halt. Especially after seeing it the firs time, which "Farpoint" demonstrates admirably. But a couple elapsed cutaways would take care of that, for detaching and re-attaching... the fact they did warp core breaches umpteen million times put the kibosh on the momentum a few times too...
I'm amused because I've been watching early Super Sentai (the basis of Power Rangers) lately, and in every single episode they drag the story to a halt to spend a minute or more on the stock-footage sequence of the giant vehicles deploying, unfolding, and combining into a giant robot. It's all about what you're used to....