Outside in the yard without any chance to get other people sick? Not really a big deal. It can actually be helpful in a lot of cases to get some sun and fresh air while sick.
Outside the house at the store, library, playgroups, school, parties? NO! Not only is the kid likely miserable, doesn't want to be out anyhow, and has a weaker immune system and can contract other illnesses, but the choice to bring a sick kid out in public can mean that an immune compromised child or adult is severely injured or killed as a result of contracting the illness. (Or, any child can get sick, causing other families potential financial hardship, work problems, school issues, etc.)
So, outside the house or in a situation where the child won't infect others or won't be infected with something worse, not a big deal as long as the child is up for it.