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?
If we knew what it was we were doing, it wouldn't be called research, would it?
— Albert Einstein
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