About the part that confuses you:
Well, yes the 'backscratcher' presents the positive connotation of being an opportunist. It is more often taking advantage without regard of principles or consequences, however it could be also exchanging mutual benefits on a basis of principles.
Yet to be an opportunist in a positive way even indicates the occurrence of circumstances of possible benefit and seizing them immediately for ones own good. On a language site here the whole idea is built on exchanging languages, it is not like an out of the blue advantageous circumstance that one could make advantage of.
So linguistically you are not an 'opportunist' neither a ' backscratcher ' when you exchange languages here :)