Mine are divided, and I completely agree with your side of it.
1) Complete impulse. I wanted another betta. And I am quite "younger", so $5 versus $70...
However, I divided a 5 gallon. I somewhat regret this decision. The only tanks from now on that I will divided are 10 gallons, or that gorgeous Petco 6.6 gallon...although the latter is the least likely between the two to divide.
2) I would disagree with "ruining the look of the tank." If you do it CORRECTLY, the divider can be gorgeous, or simply, plain, and aesthetic. Mine is black and white. Very simply, non-obstructive, absolutely not a bother. However, to each their own opinion. But my mind is slowly changing, and I do like a look of a full tank.
3) I don't like the thought of adding more fish with my bettas. Not that there is ANYTHING wrong with it, I simply like seeing them on their own, throughout the tank. Perhaps a snail or two would be adorable. I wouldn't want other fish distracting me from a betta. Maybe you catch my drift that...I like bettas? Haha.