What's even easier than measuring with a kitchen scale is just throwing the entire can in and calling it good. That's often why these recipes used "boxes", "cans", etc as units of measurement in the first place. By converting to standard units you're increasing the amount of effort needed to actually make the dish. It might be more in keeping with the spirit of the recipe to just substitute similarly-sized cans or boxes, even if it's not quite the same taste. It depends on your priorities I suppose. (Though either way it's probably good to include units for the sake of clarity and reproducibility: e.g. "one 16 oz can" rather than "1 can".)

Agreed, I wouldn't say XXXX grams of Hershey's chocolate syrup, but I do want to know what size "a can" means.

On the other hand, for things that you would always measure (or need to do to sizes changing) like flour or sugar, I want that in grams for easy measuring. Even chocolate bars, might be easier to just say how much you need since getting exactly what you are looking for might be difficult/impossible.