I'm not 100% in agreement. I think you should quote prices according to the project or client in question. Some big long projects call for billing by the week or month, many work well with days, but sometimes you have someone who just needs something quick, and you can't reasonably charge them for a full day because they realize full well it's not going to take you that long. They're going to be happier with a high hourly rate and feel that you're being very careful to only charge them for the hours worked. This is, after all, how lawyers work, amongst other professionals.
IMO, at least - I think it depends a lot on what sort of clients and work you're doing.
I may be biased towards high-end consulting, but my response to this is "1 billable day, minimum". Flexibility is something that is valued by clients, which means it has a price.
Strong relationships are something that has value to me, so if I reasonably can, I'll tend to just do stuff that takes an hour or so for free. Every consulting proposal I write takes 2-4 hours from my life, minimally. The last one I wrote required a 3 person research project over several days. We'd never charge for a proposal, so I'm not going to freak out over 1-2 hours.
(Dave, Jeremy, and Cory might disagree with this assessment.)
If you're ever wondering why I have zero sympathy for designers complaining about "spec work", there you go.
This is typical, and is expected by enterprise customers. Additionally, it's expected that 1 billable day could end up meaning anything from 5 minutes telling a staff tech which wire is crossed, or 15 hours working an exceptionally challenging problem. ymmv, and it all evens out in the end. If you feel like you're getting the shaft over the course of several projects, that's when you know it's ok to raise your rate (or that you need to improve your skills).
IMO, at least - I think it depends a lot on what sort of clients and work you're doing.