SIMBL is an extension used by third party developers of hacks and/or modifications to OS X such as Pithhelmet and others. To me it appears to create a hole in El Capitan's security wall that could be exploited by malware developers.
In short I would not be keen on installing it either. Why put up a wall to keep out intruders and then intentionally knock a hole in it?
"All you've got to do is own up to your ignorance honestly, and you'll find people who are eager to fill your head with information" --Walt Disney
