Recently I've noticed a sharp increase in the use of "be" as part of a command, especially on the radio: "Be well, do good work, ..."; "Be caller number seven and you'll win ..."; etc.
Of course, this mode of speech has been around for a long time, and there are countless classic sayings that apply it: "Be my Valentine", "Be prepared", "Be a man", and the like. But maybe the pace has accelerated during the past few decades, after the slogan "Be all that you can be" caught on? I don't know.
But in any event, if you don't want your prose to sound clichéd, be careful ...
(see also MyOb (18 Aug 2002), ...)