It does add a level of uncertainty, which makes planning and preparation more difficult. It's hard enough to judge rations and torches when you're not certain how much time you'll be having need of them. If you also don't know how much use you'll get out of a unit, the only calculation you can make is worst case*, and then you end up most of the time hauling way more stuff around than you can actually use.
Another effect is that bad results can easily be attributed to bad luck, instead of the players having planned poorly. In most cases, you want players to figure out what mistakes they made and learn to improve their planning. I also believe that it's actually less frustrating to fail when you can see that the situation could have been overcome with a different course of action, rather than suffering consequences that are no fault of your own.
(*Yes, you could do a statistical analysis to prepare for a secured supply within two Standard Deviations, but that's not the kind of play almost anyone wants to play.)