Moral Issue
I'm working on a "project":
I decompile maps and cut them into little pieces to get a lot of "prefabs". Would you consider that as stealing ?
I don't do it because I'm too stupid for mapping (in fact I'm very good at the "technical" stuff) but prefabs make everything easier.
David
To tackle the moral issue first: [copying] someone else's work without their permission is stealing. Full stop.
Also, the de-compiling tools never give you the same result as the original VMF file. If you haven't already, have a look at the info about de-compiling on the Valve developer site.
If you want prefabs for common puzzle elements, check out Aldez's prefab collection.
Edited: changed "using" into "copying"; basically what Remmiz says. Also made the link more prominent.
Other Portal 2 maps: Medusa Glare
Portal 1 maps: Try Anything Twice | Manic Mechanic
Decompiling and examining the map: Fine.
Decompiling and copying the map: Not fine.
Decompiling and copying the map: Not fine.
This.
If you want prefabs, search this site's download database; someone (can't remember who, sorry) created a pack of prefabs.
Of course, you don't learn anything by using prefabs. Also, they will probably be fitted to the map they originally came from, so it may be hard to adjust them for your map.
"Duct Tape is the answer."
Thank you.
It's not a problem anymore because I asked the creator of the map. 
Excellent.
Don't forget to give that person credit when you publish your map!
Other Portal 2 maps: Medusa Glare
Portal 1 maps: Try Anything Twice | Manic Mechanic
It's not a problem anymore because I asked the creator of the map.
Really? Congratulations, you are on your way to become an ethically correct map maker!
"Duct Tape is the answer."
Is that the politically correct term?
Bah, like politic people are concerned about ethics. All they want are the most votes.
"Duct Tape is the answer."
How right you are.