You can also use it for getting tips to your own level designing. I've done that a lot with open levels too when I was in that age. (I didn't know how to make shootable blocks until I saw it in a JCS FAQ/some levels.)
Personally I would NEVER open a level to edit it! I surely respect the users creation, and of course the originality of the level. The bad thing about editing levels is: 1. If the author has denied permission to do so, you are breaking the agreement (the author also has a part in it; posting a level online for example is also a risky descision, and besides most authors doesn't actually post such called "end-user agreements", but anyway I think it seems logical for everyone to obey this rule without a txt document telling it.). 2. Post it on the Internet in your own name, that's really rude! But in most cases it is allowed that you can edit the level for own use, no distribution(/sharing, with fake arguments). (Many game publishers with editors included in some of their games allow that. Also some level authors.)