Like title.
If you want to replay for example a AA replay which was recorded with 1.40, and you only have 1.41 installed, it will begin to start, but then report missing files and crash.
As there are so many different versions of the different mods, i dont think the user should have to check in the mod list (which he can only view in the "host game" menu) weather he has the mod installed.
Also, this makes viewing demos by just doubleclicking a downloaded demofile a less enjoyable experience.