Ugh. You wouldn't think so, but I caught a case of this in production, using millisecond timestamps as unique ids for browser analytics. The developer felt that even though two users could in rare cases click at exactly the same millisecond, their clocks weren't likely to be in perfect sync with each other, and that this "additional level" of randomness made a collision virtually impossible.
I suppose it can be counter-intuitive, but people really need to learn about timer precision... and, more importantly, statistics (basics, at least!).
I absolutely hate the 'combinatorics' part of statistics, but distributions, etc. seemed pretty learnable to me... but then I like continuous math, so...
A second has 1000ms. If you have more than 1000 users doing things per second, there MUST be a collision.
... also, it Technically depends on your throughput (something something queue theory).
Even with a small number of users per second (like 20) it becomes likely to happen often.
I think that the clocks not being in sync doesn't increase randomness in any way, as two clients who didn't click at the same time might still get the same value.