When I ordered my Rodriguez fixie in 2009, I ordered it with 2mm of "negative" slope - it's slightly lower in front than in back. The idea was to allow for different fork heights keeping the top tube relatively level.

Besides traditional aesthetics (I grew up with level tt bikes thru the 70's - 00's), the level tube is an advantage when you are stopped and you want something to rest against. With a level-tt bike, you can:

a) stand astride the bike with one foot clipped in, and rest that thigh on the top tube without having to bend your supporting leg too much. You can then rest your elbows on the brake hoods. Quite comfortable as a resting position!

b) you can get off the bike completely and use the bike as a portable rail. You just turn and sit on the top tube while you hold a brake lever with one hand (you can even squeeze it to keep the bike from rolling), and rest your other elbow on the saddle! With a slanting top tube, the sittable portion is either too low or you slide sideways on it.

So yes, there are good reasons for the level top tube besides the aesthetics.

Luis