Valve has always produced box art in a strictly logical sense.
Left 4 Dead in particular - they're simply going, "it's a zombie's hand (dead), he's holding up four (4) fingers, and it's his left hand. Left, 4, Dead."
Same thing's happening here - it simply portrays the facts. There are portals (portal). There are two (2) of them. There are also two (2) robots. Portal, 2. It's not artwork, it's just a visual representation of the information inherent in the title (and subsequently, the game).
It's like you said - functional. I wouldn't go so far as to call it "ugly," it's very well rendered and compare it to other Valve box-arts (especially Half-Life 2 and the Orange Box), it is well-staged and such.