Sense of the "Vessel has to say 'yes'" system?

I thinkt he intention was to highlight the purity of the angels. They won't/can't take a body without it willfully giving up it's body. Even Lucifer has to have his vessel say yes.

In Castiel's case we are shown that he had his vessel go through several "trust buidling" exercises to get him to say yes. Lucifer, however, preyed on the vessel's weaknesses to get him to say yes.

So yes, it's a loophole in the sense that the angels can torture their vessels into saying yes, but most angels are pure enough that they wouldn't do that.

I think of the "possession" either demonic or angelic as a very intimate thing. To have them inside your body and mind with access to your memories and thoughts.

As such a demonic possession is almost a form of rape, whereas the angels have a much more loving relationships with their hosts.

That's not to say that an angel can't attempt to manipulate the vessel in order to gain their consent (Lucifer and Sam/Michael and Dean).