PSL isn't a list of dangerous sites per-se.
Browsers already do various levels of isolation based on domain / subdomains (e.g. cookies). PSL tells them to treat each subdomain as if it were a top level domain because they are operated (leased out to) different individuals / entities. WRT to blocking, it just means that if one subdomain is marked bad, it's less likely to contaminate the rest of the domain since they know it's operated by different people.
Marking for cookie isolation makes sense, but could be done more effectively via standardized metadata sent by the first party themselves rather than a centralized list maintained by a third party.
Informing decisions about blocking doesn't make much sense (IMO) because it's little more than a speed bump for an attacker. Certainly every little bit can potentially help but it also introduces a new central authority, presents an additional hurdle for legitimate operators, introduces a number of new failure modes, and in this case seems relatively trivial for a determined attacker to overcome.