The only advantage to this app I can see is that it makes it possible to enable and disable the use of SIMBL and SIMBL plugins in the ~/Library/Applications Support/SIMBL/Plugins folder. If that's what you need to do, then EasySIMBL can be useful. Otherwise, installing SIMBL is easy enough with the standard SIMBL installer - and the package includes an uninstaller as well. By the way, SIMBL is a scripting addition that, when installed, can be found in the Library/Scripting Additions folder.
FYI: While mucking about with EasySIMBL I found that I had an old version of ColorfulSidebar in the root SIMBL/Plugins folder, which may have been conflicting with a newer version in my user SIMBL Plugins folder. So be careful you don't have other SIMBL plugins installed in both locations.