Also, further complications can arise with regards to normal vs. extended/unrated cut versions. So in theory, there could be 4 (or more) different possible times for the one shot.

Yeah and divx versions also have different time tags, as well as Director's Cut editions etc.
But as I said it is not a so big issue, and still better than nothing.

"And what if the pictures are replaced by an European user, the PAL tags may be more useful than NTSC ones."

It's not even really an NTSC vs. PAL issue. For someone in Europe, a bluray and PAL DVD will still run at different speeds, causing the same issues.

Well as I am one of the few who puts time tags (and most of the tags come from me), sorry for putting too many PAL time tags
It is better to have a "wrong" one than none.
And what if the pictures are replaced by an European user, the PAL tags may be more useful than NTSC ones.

For replacing pictures, the best way is to sort vehicles by time tag (appearance) and then fast forward through the movie and capture them in the right order. If you find a vehicle further than the one you wanted, you can then search with the time tag or just between the previous and current captured scene.

My suggestion is that PAL timestamps should be not accepted, since the NTSC speed is the "correct" speed. This has already thrown me off several times, for example, when trying to replace SD shots with HD, often I find that the timestamps are obviously for the PAL version since they're a few minutes off.

It is rare that time tags are entered by different people.
Either only a few cars added by a person will have time tags, or most of the cars will have except those added by other persons.
If someone adds time tags to a movie that already has them, and they do not match, he should either replace all tags or not put them on added cars.
There aren't many users who set them so until now it hasn't caused much problem.

But the problem is, all the time tags aren't going to be consistent, because one movie might have several contributors - some of whom might be using NTSC sources and others PAL sources.

As long as time tags for all vehicles of a movie are set from the same version of the movie, it allows to sort vehicles by order of appearance in the movie, allowing to find more easily the one searched and a good idea of what is visible in which scene.
And it still gives a good idea on where in the movie it appears.

With regards to the time of pic for screen grabs, I question how useful providing this is, considering that it will vary depending on whether the grab is taken from a PAL or NTSC DVD (note that with blurays, this isn't an issue). PAL DVDs run at a 4% higher speed than NTSC, so that if I take a screen grab from an NTSC DVD at the 25 minute mark, the same frame would appear on a PAL DVD at roughly the 24 minute mark. The further into the movie you get, the greater the discrepancy, i.e. at the 120 min mark, the difference would be 5 minutes (120 vs 125 min). Seeing as this is the case, is it even worth bothering putting it in?