Personally i think for the length of time I have the cube open, in a consealed draft-free area im quite safe. I quickly add my hops, I then reseal the cube once more and purge all air out (as much as possible) of the cube as well.
I have yet to have a batch go belly up (6 so far with this method). The chances of it picking up something would probably be less than that of those who religiously no chill in their fermenter h34r: .
There's cubing and cubing - 80 percent of the time I use cubing as a cooling method and pitch ASAP. On the other hand I cube a brew into four 5L wee cubes and store them for up to 2 months and add a cube to my regular partial brew.
Sure you go to extremes with a cube that you want to store for months, as you would also do with a jar of homemade pickles or preserved cumquats or whatever. However doing things to a cube that is only going to last 24 hours isn't any different to opening a fermenter and chucking in a hop teabag etc and has probably fewer attendant risks due to the high temperature in the cube.