Is it a pain? Yes. Is it legal? Mostly. (Check your local laws, I’m a blogger, not a lawyer). Does it work better than using a cracked key that resets your plugins every week? Absolutely. Don't be the person who explains to their boss why the company’s internal vulnerability scanner is reporting CVE-1999-0001 (yes, from 1999) because your cracked license stopped updating.
But here is the life hack nobody tells you: Most networks don't need more than that for a spot check. nessus license key
I Found a “Free Nessus License Key” on Google (And Why I Immediately Deleted It) Is it a pain