We love Wrike's proofing tool for our larger clients where it makes sense to add them as collaborators where they login and see all of their open projects. However we have a good amount of one off clients where we may only do one project for them. It really doesn't make sense for us to make them create a login just to proof one project. We have tested some other software (that misses the mark in a lot of other areas) however they give you the option to simply copy a link that will take a user to your proof where they can proof as usual without having to create an account. Their comments show up in the account and business is as usual. It works really well for projects you still want to keep organized internally yet allow one time clients to comment without the hassle of signing up or showing them more info then they need to see.
Just some food for thought,