Since I don't think there are too many, if any, examples where there are vehicles rolling off an assembly line, I assume that all the craft have been through some sort of use, especially during the Clone Wars. I don't recall an instance during any of the battle scenes in the Saga where there were "factory fresh" looking vehicles engaged in combat.
For me, weathering is a preference if not a must.
Just my 2¢.
Well, to be fair, there's a lot we don't see in the movies that still makes it into plastic. I sure don't recall seeing a firespeeder pilot on screen, but I know someone had to be piloting those ships. Likewise, it makes sense that at some point, all of the ships looked clean and unweathered. We do see things like the troops deploying at the end of AOTC and all of those clones and vehicles are minty fresh. In the EU world, all the characters use sonic showers, so you could make an arguement that the ships are just cleaned wehn reloaded, even further cutting down on any actual weathering. Add in the rationale above as to why they probably wouldn't get that dirty in the first place, and I think it makes better sense to not rough them up.
I've personally always felt that the Empire was meant to be ridiculously spotless clean. Look at how pristine the innards of the death star and star destoryers were, or how shiny all the sandtrooper armor was. Everything is polished clean and cold and machine-like. That was always symbolic to me of how the Empire was the antithesis of nature and individualism, whereby we see the Rebels making bases out of ice caves or teaming up with teddy bears in a forrest.
Plus, releasing them clean means you can weather it any way you like to create your very own one-of-a-kind collectible! I would much rather have the facory fresh clean version and am often dissapointed when I see weathered ships or figures, so luckily (or unluckily) they're catering to us both every now and again.