I think we're saying the same thing then.
So what's left is to answer GP question, which nobody has done:
What are the use cases for using the repl directly rather than through something like SLIME?
You answered "both" which I'm sure is correct, but I'm curious as to specifically which usages you find better directly through the repl. The only reason I can see is when you can't be bothered to (or are unable to) start SLIME, otherwise even to evaluate small expressions I'd rather have them written in a file to easily keep a history of them or edit them.
I also know people who never use tools like SLIME and prefer just using the repl for simplicity.