It's useful as an upper limit--if you're leveraging past what Kelly suggests you're almost certainly overleveraged.

In theory, Kelly is optimal--if you knew the exact probability density function of your returns, it would give you the right leverage to take.

In practice, you're always playing with risks, some you're factoring into your models, some you're choosing not to because they're intractable, some you're not even aware of until they occur. The most basic premise--today's returns will be a function of hypotheses that I've derived from looking at past observations--is an approximation at best.

This mismatch between model and reality can lead to expensive lessons learned when using the full Kelly model, so often traders will "half-kelly" or something like that, to incorporate the basic idea of risk scaling proposed by the Kelly model but with more safety margin.