I completely agree! This aspect of the interface is very annoying.
Say I've got forty workers hired at a base and I want to add 10 more. I need to click the down arrow thirty times (if you're going to do it this way, a scroll bar here would be a huge improvement) and then click ten check boxes to select the workers. Even then it's difficult to know if I have my count right. So after forty mouse clicks I've finally conveyed the info '+10 workers', and I'm not even really sure if I've selected precisely ten or not.
At a minimum: Display a total workers (scientists) count so I can see how many I've got while hiring and firing. Add in a scroll bar so I can quickly navigate to the bottom of the list.
or even better: Hire workers (scientists) the same way you buy/sell items.