Hey Marc, Good post and discussion. A handbook is an inevitable and necessary evil of business growth. However, as Lee mentioned (and as you pointed out with the need for vacation policy) there is value to some properly designed guidelines. Yes 'HR weenies' can mess up the party. Those schooled in the traditional HR functions are too often tied up in routine and standard or "best practice". If suddenly your cool startup has a policy for a start time, a progressive discipline policy or a detailed cell phone policy - then someone messed up the party.
But I think its needlessly negligent to have nothing in place. You want your startup to have the right foundation and to have some structure to refer to (to encourage vacation in some cases like yours Marc, or to set 'general' expectations that you can point to when an employee is abusing them). Employees have questions (Holidays, vacation time, even if reviews will happen) - give them some help.
Perhaps most importantly, employee guidelines reduce your legal liabilities. You don't have to do a 3 hour orientation yawn-fest or have a heavy bound book for employees to shove to the back of their file cabinet. But, depending on your culture, you can address some key practices (or typical concerns), you can list the federal laws your company stands behind (like non-discrimination and harassment), and you can demonstrate that employees are expected to adhere to federal and local laws. This addresses concerns Lee brought up in his comments, and it doesn't have to move you into the 'small business category' or into inflexible and stifling workplace. Boom... a LOT more protection than you had and a tool to lean on if you need to deal with an employee gone bad.