I'm sure it is imprudent.
And against my principles as well, if I did it without at least engaging with the subject and confirming their agreement.
I'm not sure that it is always unethical, but ethical issues should be addressed.
But I can't think of any reason it would be illegal, although in places like Germany or Quebec where there are special, more restrictive statutes that deal with privacy rights, claims for damages might arise.
As for the model release, they tend to be required by publishers, because they protect publishers from potential litigation, but their main advantage is that they confirm consent.