I did not know about space bar for scrolling, but I usually use the mouse wheel for that. The space bar doesn't always work. For instance, right now the space bar is putting a space in this message. What if I want to scroll down right now to the bottom of this page? I just reach for the mouse. I can still momentarily cover most of the keyboard with my left hand for typing. Also, if I go to the top of this page and hit space bar, it will scroll down the page until it reaches this box of text that I'm typing in. Advantage mouse.
The mouse is still quicker for so many other important things. For instance, let's say I mkae a typo. Oops, I misspelled the word make. What's the quickest way to get back to it? The mouse, at least I think. Thing is, anytime I have to reach for the mouse, I will momentarily keep typing with my left hand, even if it means hitting characters normally reserved for the right hand, so I don't really lose a lot of time. I certainly save more time by being able to instantly jump to where I want with the mouse. Then I will revert back to normal two handed typing until I need to use the mouse for something. For making text bigger or smaller, I use ctrl +mouse wheel.
For user name and password, tab and enter key are usually the best way to go. There are times when I'm not confident that the enter key is going to activate the button I want, so just to be sure, I may use the mouse. In short if you're comfortable occasionally typing with just your left hand while mousing with your right, this changes the rules somewhat. With the mouse there is no question about what action is going to occur. Unlike the space bar or enter key, it's not like something different is going to happen depending where you are on the page.