Because I usually wear navy blue suits, I thought it would be a no brainer to just purchase a beater navy dress trousers for days when I don't feel like dressing up in a suit. But if light grey is more versatile, then I'll go that route. I haven't thought of it that way.
Rudals / The Noodles, no sense rehashing the navy odd trouser debate. There are a few threads on it already. That said, I think even most who are okay with navy trousers would agree that a medium grey trouser is more useful to have as your sole pair of odd trousers.