Never could understand SWIG much. Cryptic interface language.
If you intended to write a library that needs interfaces in multiple scripting languages, then SWIG might lessen the workload. Or if you are wanting a Ruby interface for some library already written that has SWIG interface files, then again I can see using SWIG.
But for an extension that will be written specifically for SketchUp, since it only uses Ruby, I cannot see much use for SWIG.
Link Preview Image SWIG - Wikipedia favicon(en.wikipedia.org)
Link Preview Image SWIG Tutorial favicon(www.swig.org)
Link Preview Image Simplified Wrapper and Interface Generator favicon(www.swig.org)