Unless you sail in flat water, with a perfectly clean bottom, brand new sails and have perfect trim, they are totally useless. If you have all these things, they are modestly useless. Besides, getting to know how a new boat sails in various conditions is part of the mating process. Embrace it, don't overthink it.
Ah, well, not really. In everyday sailing one doesn't always have a "clean bottom", few actually have "brand new sails"; and, most can't achieve "perfect trim". One can, however, generate a set of useful "polars" for one's own boat sailed under "normal conditions" as to bottom, sails and trim so long as one can get a day with relatively steady winds in a body of water with relatively little tidal current, or, one can determine the set and drift of the current which isn't so hard if one simply lays ahull, absent sails, for awhile and keeps track of one's COG and speed as one "drifts" with the currents. With that, one simply sails a set of courses at reasonable intervals on each tack/gybe from DDW to as close to the wind as one can point and records one's boat speed, apparent wind, true wind as computed by ones instruments or manually if necessary (assuming one has a reasonably will calibrated Speedo). One would, of course, want to do this several or more times in various true wind speeds but it wouldn't take much effort to generate a reasonably decent set of polars for say, 10 to 25 knots, for each of one's head sails, assuming that one has more than a single sail and might switch sails to account for anticipated conditions on or during a given passage, tho' most likely do not but adjust a given head sail for the conditions as they evolve.
In our case, we have very elaborate polars for our boat as it was developed for racing but I found the foregoing approach quite helpful and fairly easily done with a pad, pencil and an Excel spreadsheet. Making up one's own polars as described is also more effective in some respects as they are specific to one's own boat which may have performance characteristics quite apart from that of a "test boat" for a given model/design.