Well, what I think about the promo code depends on the hosting provider. If it is someone offering 20% discount bydefault, I am damn sure they have their prices already hiked. A sales strategy, just to attract visitors.
Check Web Hosting UK or other such companies, they hardly offer any such discounts on Shared plans because their prices are already low.
Promo codes can be useful for higher plans like Dedicated Server. If someone thinks of using Promo Codes on Shared Hosting, (s)he should simply consider Free Hosting