There are use cases where you don't have execute privileges. In those cases the .sh-extension makes it clear that you can do `bash script.sh`. If you don't use an extension you wouldn't easily see that that was an option.
No, it doesn't. The extensions are usually too inaccurate to rely on. That could be either a Bourne or Bash script, meaning it could either fail at some arbitrary point during run if the wrong one is used, or just subtly, critically change some output. Much more true for Python scripts.
