[zerr]

Well, when you charge per day it is assumed you charge for 8 hours, isn't it? For small tasks (2-3 hours) it can be better (rounding by day), but for long-term, undetermined timespan tasks (several months/years+) charging hourly also gives you a freedom to not to work 8 hours per day.

[ElCapitanMarkla]

Depends on the client. Typically a day rate is a good chunk of change so it's a good way to get clients to think about things a bit more seriously / show a lot more respect for your opinion/decisions.