I've set a logo in a wide grotesque typeface that only comes in small caps. The client has requested to see a variation of the logo with upper and lowercase letters, and now I find myself in the unexpected position of having to explain that some fonts don't have lowercase letters without making it sound like I've chosen and incomplete or inadequate typeface.

Any suggestions? I admit I don't fully know the history of the style, and suspect there is likely a historic reason for it, but I don't want to have to give my client a history lesson. Any tips on a clear, concise way to explain (and justify) the use of caps-only typefaces besides "it looks cleaner/bolder/better"?