I think a lot of people have sharp eye too, but when you watch a movie, you don't pause every frame to view for a minute, then forward to the next frame and do the same, do you? you don't break the flow of the motion.
What I meant was if you render an image in 4000 x 3000 pixels, of course you can see all the details, but try resampling that image in photoshop to 720 x 576 pixels, will you still see those details? Nope! they became a blob in a larger pixel. What I was trying to say is you can find the right visual balance to fit the TV/video resolutions which means you don't have to waste any unneccesary time for some details which are not going to hold anyway.
Also, capturing stills from TV/video will never give you a good quality to print, most TV/video are interlaced (which means two images, e.g. frame 1 and frame 2 combined together with lines between them), again, they are only 720 x 576 resoultions, will that be good for wallpaper?
I have done many animations at work which time is always against you depending on client's budget as well. I have done exactly what you said before, create every 3d objects in highest resolutions (even the objects are only in the background) with loads of fine details, and took weeks to render, but the result is alot of the work I put to the finest details do not show up in the 720 x 576 resolutions.
By learning from the experience, you will know how to plan you animation before hand such as will the object be seen in close up or not? (close up will require higher res modeling) if not, you could cut the modeling time by half, and if some objects can be texture map rather then real models or bake the GI textures to objects could save alot of time, which the end result still be as good as the original GI scene. There are a lot of tricks can help you cut the time and still render in good result.
It may be I am in the trade where delivering the good is more important than not delivered at all, where time and quality got to be compromised. Who don't want to have a large render farm and ultra powerful machines to do 3D? When you don't want to or can't wait for the slow renders, you will have to seek alternatives. This is only my experience, of course you don't have to agree.