Generally, yes, it is safe to delete them so long as you don't get some error message about the file being in use. However, aside from some obsessive compulsive anal retentive mentality about cleanliness, there's really no point to clearing them out. They don't hurt anything by being there, disk space is cheap and plentiful that we don't need to worry about a few stray MBs, and you can cause some problems by deleting a file that was still needed by some program.
As to why Windows doesn't clear them out... To Windows, they're just like any other file. They aren't any more or less significant than some word processing document you made, or the main executable file for that new game or software package. It's up to the programs to clean up after themselves, and as you can see, it's not something a lot of them do. If you think of it from the standpoint of the Windows programmers, how exactly are they supposed to be able to tell which temp files can be cleared out and which can't? Especially for programs they've never heard of, or haven't been released yet. Just because a file isn't in use at the moment doesn't mean it won't be in use later. The safest course of action for them is to just leave it up to the program developers, who frequently drop the ball.
Still, disk space is cheap and plentiful, so unless you have over 1GB of temp files, I wouldn't worry about it. Just leave 'em be, and spend your time on other things. Fun things.
Can someone tell me if it's safe to delete .tmp files in the Windows \Temp folder? Which ones are actually trash, and why doesn't this umteenth version of the Windows operating system (xp)trash them on it's own? I'm also wondering about the files in my C:\Documents and Settings\All Users\Application Data\Microsoft\Windows\Game Explorer folder. It's chock full of files with alphabetical strings as filenames generated by windows that contain shortcuts to the same gamepage. I'm stuck on an old pentium III for now with very little disk space and need to ecomomise it's disk space.
Any help is welcome.