I'm sensitive to solvents so I paint solvent-free, using either linseed or walnut oil as a medium. I've painted both with "regular" oils - Rembrandt, Gamblin, Winsor & Newton - and Geneva oils. Both are great, although nothing beats the smell of Geneva paints and their ease of use.
I've used solvent-based mediums on a couple of occasions in the past and don't see a big performance benefit over an oil-only medium. I know there's got to be a reason so many folks swear but I don't see the point.
Could an experienced oil painter who has a lot of experience with solvent-based mediums enlighten me?