the problem is more specific than the second employer getting non-zero value. It's getting ongoing value, under the table, direct from a present competitor -- harming the relative competitiveness of the first employer.

The value of this, of course, depends very much on the specific industry and project. It doesn't really matter if your guy who builds generic websites for your clients also builds generic websites for a competitor's clients. That doesn't materially hurt the first employer. But it can matter a lot if your guy who you are paying to develop cutting-edge algorithms is deploying those algorithms to the competitor you're trying to beat. Instead of gaining an edge worth $X per year, your company instead gains only part of that advantage because at least one competitor is cutting into it.