Let's say I only want to animate location and rotation, what difference would it make if I keyed the scale as well but kept it uniform throughout the animation? Does it affect animation's performance or size of the blend file?
The main reason is simplification. When you're working with a team or if it has been awhile since you looked at the file. Trying to figure out what has been animated and what hasn't slow you down. Also when you start animating several objects they will be in the way and clutter up the dopesheet/graph editor. Another reason is if you start doing reusable animations they can cause problems and if you're not in the habit of cleaning up unused channels this will become something that slows you down. Memory and file size are relatively small and aren't that much of an issue. Unlike zero weights on an Armature.
There are already some good answers here, but I will just reiterate a few.
The #1 reason is to keep things as simple as possible.
It will just add clutter.
It also helps the animators see what is important.
"Hey this scale channel is unlocked what does it do......oh nothing"
It does add to the file size, which won't be much unless there's hundreds of controls (which is often the case), but file size isn't a big issue.
It can slow down animation playback depending on what the computer has to draw. The more windows you have open and the more curves that are visible, the harder your computer has to work to playback the animation. Showing curves during playblack does slow it down - so having curves that do nothing is just an extra ball and chain the computer has to draw.
The display of curves has improved dramatically over the past 2 major releases, but keeping the playback as fast as possible is something we always want to do in animation. If animators can't view their work in real time, they have to playblast. This slows them down and adds significant costs.
Hope that helps clarify things.
