I could see a straight weight oil being best in an application where an engine was run continuously for long periods of time and not shut down and restarted very often at all.
Perhaps a generator of some sort?
I could see straight weight oils being best in an application where they never see low temperatures (due to climate or preheat), where the oil is subject to sheer (all applications, but especially motorcycles) where the alleged high startup wear (alleged because although I've seen the numbers quoted a lot, and they are plausible, I've never seen a source for them) is less of an issue due to the above, or due to the use of a prelube system, where the engine sits idle for long periods of time, or where the inferior (?) lubrication of polymeric viscosity modifiers is an issue.
A LOT of applications, but I don't know what the advantages (if any) are ACTUALLY, because I've seen hardly any evidence.
Either way, it's probably out there, but I havn't seen it. A lot of it is probably secret.
Someone on here recently (I thought it was in this thread, but I can't find it) posted a response along the lines of "or do you think multigrades are just a worthless marketing ploy" as if that was inconceivable.
I don't think they're just a marketing ploy, but "worthless marketing ploy" is, in a commercial context, a contradiction, like military intelligence.
If a marketing ploy is effective, it'll be anything but worthless to the people who implement it, regardless of any technical merit.