wouldn't that effectively result in the devaluation of HBD on the markets

It could but as we've seen the price of HBD is market-determined and doesn't necessarily align with exactly how much HIVE it could be converted into. As I write this HBD is worth about 6% more than its conversion value.

The haircut affects the amount to which it can be converted, but the market price is still a market price, could be higher or lower.