Let's say that my VM of choice has a tracing JIT, and it will attempt to optimize traces up to 1,000 instructions long. Let's further say that in the current version of my code, the body of my hot loop is 980 instructions long. Then in a new version I add another few operations which push it up to 1,010 instructions. Suddenly the JIT stops trying to optimize that portion of my code and performance tanks.
Meanwhile some other guy wrote his code using a VM which used this sort of "Jets" approach. It's probably not as fast or as versatile overall, but when he adds another few dozen instructions to a hot loop he can do so secure in the knowledge that all the preexisting code will continue to execute just like it did before.