They would be useful in order to see if you are sailing as well as could be expected. If Polars show that you should be going 6 knots on a beam reach in 15 knots of wind, and you're only doing three, you know something isn't right.
If that works for you, enjoy the process.
I still donít get it. A beam reach, with 15 kts of breeze, should be damn near hull speed on just about any boat. A good focus on sail trim is going to get any boat within a narrow margin of a polar prediction. I donít see the polar being the right tool, if one canít get close enough.
The real advantage of a polar, as I see it, is to determine optimal VMG to a waypoint. This will be less than optimal, without wind instruments.
You can try to make your own polar, but theyíll be specific to ones skill to trim and condition of the sails. Generic polars, as others have stated, are fairly unrealistic. I know the sails I just retired were doing just fine deep downwind, but not pointing worth a damn.
Iím really good with everyone sailing their boat any way that makes them happy. My encouragement is to look at the windex and sails and not in the book. After a while youíll know whether youíre sailing efficiently, without having to check, but you do you. Enjoy your boat. Cheers.