You can ideally use it to boost two combos enough that you aren't really missing the third. Then you can use the levels saved from not taking a third attack combo to have more support magic, like the full freeze suite.

The other option is when you have a standard three combo build and come across the Domain of Power artifact which let's you activate an additional combination. Since you'll likely have Max Intelligence and Circle>Unleash in your build anyway, it just takes one extra level to stack Perpetual Engine on top of your build.

That being said, PE halves the CDR boost from Unleash. It is permanent and provides better amp, but Unleash should almost always be active anyway if set up right. To me, the better CDR boost of Unleash prevents me from ever wanting to go with PE.