For example, 3 defending INF can be expected to get one hit per combat round. If you send in enough planes to ensure you’ll kill all 3 enemy INF, you only need to send 2 INF in to secure the territory.
According to probability, yes, but in practise, if you really need that territory, I’d go in with atleast 3 inf. Hitting 2 with 3 inf on def is not too uncommon. Hitting 3 is rather unlikely.
On top of that it also depends who will earn from a war of attrition. If I do, I go, no worries. If I don’t, I won’t, unless it’s for clear strategical and tactical purposes (like your opponent not being able to land planes in a stack next to your Capital).