If it's meat, I'll brush the meat with it as long as it has time to reach cooking temps. In other words, not right before I pull the meat off. I've seen another member cook down the marinade and use it that way. I don't reuse it for marinating, but that might just be me.
I save my veggie marinade for a couple days though. I love marinated, grilled veggies, so I'll reuse that for marinating another night's dinner.
Give us this day our daily bacon.