Depends on what you mean by "Some Air" If I were designing a car I'd put "Some Air" into the defrost system on AC but not much more than 10%.
As the other reply said, Defrost is the default should the vacuum system fail, but that will be most all the air (100%) And if you have an air dam (Door) that has a bad seal on it, that would allow air to flow past it as well.
If it's only a little bit of air in the defrost system.. I'd not worry about it.
Nothing adds excitement like something that is none of your business.
2005 Damon Intruder 377W Radio Active as WA8YXM