When I set a reminder from within Agenda, I almost always have to change the suggested time to »all-day«. This is because I (and I guess I’m not the only one) have vaguely in mind what’s happening on what day, but rarely the finer planning. As I use BusyCal as my main calendar, I get all tasks and events in the same picture there and put the reminder task at an appropriate time later, if at all.
Of course everybody’s different, but an option to start with »all-day« as default would be quite handy for folks like me.
However, that’s not settable in BusyCal. BusyCal distinguishes between timed tasks, all-day tasks and undated tasks (via Option-Cmd-K, Cmd-K and Cmd-U).