And if they aren't with you any longer? Say, on their way to the ER via POV? If it is a deep enough lac to need further attention, it gets bandaged.
Most of the time, my patients do go via POV for lacs. But you are correct, if they require more interventions, then the lowest priority wound/injury isn't tended to instantly. Doesn't mean it can't be later during transport, though.
Now, getting into the microbiological aspects... ALL WOUNDS ARE CONTAMINATED. Whether by whatever flora is on their skin, to the material that caused their injury, to the ground, the air, other surfaces. This is why they irrigate the hell out of wounds in the ER; to cut down on the foreign material in the wound, including those lovely microbes. I say protect it from further contamination, if possible. Do I do this for every abrasion, and small lac? Not all the time. But if I have the time, I certainly will attempt to.
Why not? Your patient may be more comfortable for it also. Getting it covered can reduce pain from the air blowing on it. And my previous arguments of microbiological control, along with the possibility of it bleeding later. Why not prevent a little? Just because we are the ones who deal with things as they become issues doesn't mean we shouldn't work to prevent and issue from happening, or becoming worse, IMHO.