that's not true -- you will eventually need many of those tiles later when the city does grow, so if the worker has no immediate jobs, why not build on them now? it's better to have them ready to go immediately when the city grows.
Yes, having the next tile a city will use "ready to go" is perfect play, but what I mean is what I just saw before I typed this, the ai had 10 workers improving a size 2 city. If they have nothing else to do, then at that time you've got too many workers
but I do agree with your point that you can have too many workers - that is another issue.
also, it's important in the earlier parts of the game to sometimes build improvements on resource tiles that do NOT provide the resource. why? because if you desperately need a few extra hammers or food or commerce, and you might not have the technology discovered yet to use that resource. so why ignore the tile altogether and leave it totally unimproved? you can always replace the improvement later for the one that provides the resource.
this is the kind of strategic thinking you have to use to beat the game at higher levels. I've seen the best players do this kind of thing and use all sorts of creative ideas to win. it's the old "think outside the box" cliche...