I am also of the opinion that the box should have the default value placed in it for several reasons, all of which will be losely based on Nielsen's 10 usability heuristics.
I picture a few different user groups:
1) Novice Users - Users who aren't too familiar with computers (reasons could be they don't have a need for it, aren't very good with technology, or anything else). In this kind of situation a user could be struggling to fill out a form and if you can it would be kind of you to offload as much work as possible off of them onto the system, in this case by filling out a few fields with default values.
2) Power users - These users won't have any trouble navigating your sites, forms, or products but will all the same appreciate the speed at which they can fill out forms by just tabbing through it without having to change anything if they don't need to. This speeds up the filling out process significantly.
These two things can both be accomplished by having that default value there and gives users what Nielsen calls "Flexibility and efficiency of use
Accelerators -- unseen by the novice user -- may often speed up the interaction for the expert user such that the system can cater to both inexperienced and experienced users. Allow users to tailor frequent actions".