"Piracy rate" doesn't seem like a useful metric; it's too dependent on your sales. I'd rather see sales numbers and piracy numbers.
If you sell 10 and have 10 pirates, that's a 50% piracy rate. If you sell 1000 and have 10 pirates (same number of pirates) that's only 1% piracy rate. The ratio is less useful and more misleading than the raw numbers.
Agreed - especially given the stated belief that the pirates are in it for novelty, many will presumably delete the game after cycling through it. As such, piracy rates go down over time (a trend noted by the OP), even if sales figures (which are more important) don't change much at all.
Of course, doesn't take away from how great it is for them to share this information at all.
If you sell 10 and have 10 pirates, that's a 50% piracy rate. If you sell 1000 and have 10 pirates (same number of pirates) that's only 1% piracy rate. The ratio is less useful and more misleading than the raw numbers.