* It positions you against the lowest-quality cheapest providers.
* It misaligns your incentives, so that you're penalized for doing a better job.
* It totally hides the cost of ramp-up and ramp-down (if you think clients push back on daily or project rates, wait until you charge them for 2 hours of "getting in flow").
* It forces you to be vigilant about time tracking lest you accidentally undercharge customers.
* Not to mention, with virtually any client worth doing business with, you (the consultant) are much more sensitive to the cost of a project than the customer is; it is a small miracle that the customer can get a programming project completed at all without potentially hiring and then firing 3 different people. So why is all the burden on you? Why is any of the burden on you? Key consulting idea: it's not the customer's money they're spending.
* It inclines you towards finicky accounting, the kind that charges a customer for a 45 minute phone conversation.
* It conditions your customers to take a fine-tooth-comb approach to project plans and invoices.
* Not to mention: it generates more invoices.
* It forces you to negotiate with clients in the worst possible numeric domain: where small deltas to proposed rates disproportionately impact the final cost.
* It obscures the final cost of projects in ways that make clients defensive, so that their immediate thought is "oh shit this is going to add up to lots of hours we better be careful".
* For that matter, it inclines your projects towards the small and away from anything ambitious.
* It impedes your own flexibility, so that you tend to miss opportunities to interleave projects or for that matter take an occasional long lunch.
* It forces you to account for every waking hour of your day in a way that daily rates don't, when we all know that only a small subset of your work hours are truly productive.
* It makes it harder for you to reasonable toss freebie work to your best clients without damaging the expected value of your time; for instance, I can cab over to a client in Chicago and spend 2 hours looking at a design with them for free without creating the appearance that my bill rate is arbitrary.