In building an invoker there are two must have edges to me, persistent earthrain(knockout protection) and absolute zero(lag). It makes high affinity in earth and ice a requirement. Looking at the other paths and edges I see nothing as nice.

Aside from learning rate, there may be other, less than obvious, benefits to high affinity. But for most elements the edges aren't there to warrant a higher affinity.

In my opinion there should be a valuable edge in every path. And a 10 affinity should make something available that an 8 affinity doesn't. It would make the class more interesting to play by giving each invoker it's own twist. It would also revitalize(slightly) a class that hasn't changed in years.

For example, the ooze path off the top of my head:
10 - an edge to manipulate items while maintaining vitriolic stream
8 - some spells might have a poison affect or do something like blister agent or abrasive powder.

Of course, it's probably a bit of just style as well. We all know CF players love the "Master" style effects. 100% skills, characters that are Masters of X, Y, Z.

Personally, I've always loved edges that give you added versatility, or encourage uniqueness based on combinations like some thief edges. Sandman, the geyser edges, some of the berserker edges (primal terror, disease carrier, etc).