I see this written often.
I'm glad to "settle" for 85/5.6, to have 15mm (just as i'm sure you're glad to settle for 16/5.6, for 8mm).
Exactly! But not... The 24-70 gives me the 15 mm FOV the crop sees at f2.8
For me it was indeed the short FL otherwise I'd buy the Tokina for the constant f2.8. No problem upping the ISO for the 8-16.
HOWEVER... I would have a problem with the 5.6 at the 85 portrait length. Narrow DOF. The 5.6 at the end of the 18-200 got me started with the L lenses. I wanted the subject separation... In the short end, who cares. If I really get into astrophotography, then I might swap for the Tokina...

