I was under the impression that if signal started balanced at one end and then unbalanced at the amp end that the shield in the cable would still help protect against certain types of inducted noise. Basically, the foil shield can still help to reject certain RF etc... whereas if the signal was sent down something similar to a speaker wire with no shielding that al 100' would be more succeptable to stuff like RF and other things en route to the amps. Basically, I run all of my returns in all of my live rigs as balanced throughout. On occasion however, often due to circumstances that are unresolvable in a reasonable time efficient manor, lifting the signal ground (cutting pin 3) is used to help battle noise issues, but this is always done at the male XLR end on a return line so that the wire is run intact until the last moment when it is cut. My experience has shown that balanced is certainly prefereable on long runs, even for line level returns to stage.
I agree running a speaker wire for a line level return is a really bad idea. There is no question that a line level signal should be shielded.
I think the results of cutting pin 3 are going to vary. I am also still not sure what signal the OP intends for this snake, so I am going to have to cover a few bases:
If you're talking a balanced line-level send, most will be either impedance balanced or active balanced. If you cut or ground pin 3 on an impedance balanced send, you are essentially just discarding the impedance balancing, which you wouldn't enjoy with an unbalanced input anyway. It is not the signal ground, so I would expect it would make no difference. The results are similar with the electrically balanced output, except those should generally be left unconnected rather than grounded, if pin 3 is not to be used.
When people say they are "lifting" a ground sometimes they mean disconnecting it and sometimes they mean putting a resistor between signal ground and chassis/cable shield. Unbalanced circuits are commonly done with resistor-lifted signal grounds. Sometimes it helps and sometimes it doesn't. One possibility with that would be wire pins 2 and 3 to tip and sleeve, and only ground the shield at the (balanced) output on the other side. Rane has some pictures of this on their site, but I've not tried that myself.
Another option for a line-level send would be a transformer balanced output, although those are fairly rare. If the transformer was connected to pins 2 and 3, and pin 3 left unconnected at the other end, it wouldn't pass any audio. You would need to ground pin 3 to drive the unbalanced connection.
If somehow we are actually talking about mic lines rather than line level returns, then anything other than a balanced snake is going to be very unsatisfactory.
Hopefully that was confusing enough . . .
