In my view, there's a way of putting this to the test. Ask yourself the question:

Are the other members easily replaceable?

F'r instance

Oasis: Yes - Non of the 'sidemen' contributed anything to the records that couldn't have been achieved by any other reasonably competent musician.

Stone Roses: No - They lost something when Reni went and would've done so had Mani left.

My favourite band of the moment, The Hold Steady, is really a duo consisting of Craig Finn and Tad Kubler. I think Bobby Drake and Galen Polivka are a stellar rhythm section, but not necessarily part of the musical identity of the band. And Franz Nicolay has just BEEN replaced, so...

Some bands are just one person, though. Look at The Cure. Robert Smith has effectively been The Cure since Lol Tolhurst was sent packing. Interesting that they would never have been the band they became without Lol, but he was in most respects utterly fucking useless, not to say an active hindrance.

Not sure the Who or Zeppelin would have been quite the same without all the classic line-up's intact.

Oops.. overlooked The Clash. And REM. Always doing that.

were never a duo. In fact, they're an excellent example of a really cohesive four-piece. Each member of the original lineup contributed in a big way to their direction - I mean, Bill Berry wrote Perfect Circle and the bulk of Everybody Hurts, among others, while Mike Mills has always been a major contributor of songs. So I'm not at all sure you're right there.

I can add a few more examples:
Squeeze (although I expect the 1st couple of albums would have been different without Jools Holland)
Sparks
Fountains Of Wayne

On the other hand I could cite Talking Heads, The Stranglers, Rockpile as examples of bands where every member counts.

I think it's surprising how many bands fit the duo premise though.

In many cases, the theory holds:

I give you:

The Go-Betweens
Hüsker Dü
XTC
Kraftwerk
Steely Dan
The Smiths