Gresham's law implys that when given a choice between two currencies to spend, people will choose the less valuable one.
bitcoin, being the ultimate form of money suffers from this effect (much the same as gold).
if i can buy something using fiat or using bitcoin ill choose fiat, because fiat depreciates rapidly and bitcoin is valuable and rare.

if I'll pay with bitcoin I'll have to go through the hassle of converting fiat to bitcoin to compensate myself for the loss of bitcoin.

this could limit bitcoin's ability to function as a common currency and instead just serve as a store of value.