Rubber bellows in clutch reservoir necessary?

Does anybody replace the rubber bellows in the clutch reservoir when topping off? I have been but it always makes a mess. Anything wrong with just leaving to out?

It's absolutely necessary.

Otherwise the cap won't form a seal in its own right.

Brake fluid leaking and dripping and blowing around won't be good for anything in the vicinity.
Besides the manufacturer designed it to have the bellows and integrate it as the only part of the seal

Work clean.
My process is as follows:-
After removing the bellows wash it and the plastic expander ring in a strong stream of water to remove old brake fluid.
Dry it completely.
Reform into the original shape.
Assemble the plastic expander ring into the bellows/seal.
Ensure the clutch fluid is down a little down from the top line(you could run it at about half full without a problem). If it's too high it will tend to wick up and start to get messy.
Wipe the top rim of the reservoir with a clean cloth or paper towel to clean up any residual fluid and discard.
Re-insert the seal/expander ring assembly into the reservoir carefully.
Replace cap and screw down firm finger tight.

This works very well for me
Brake fluid is nasty stuff

Also the fluid is hydrosorbic. So the bellows fill the negative space without using air that has moisture in it. As the level of the fluid goes down, the rubber can move with it.