See, right there, that's the trick.
You are 100% right. You don't need rules to determine the reactions. That is 100% true. You can certainly free form that and decades of gamers have done exactly that.
But, that's not quite the question is it? The question isn't, "do we need this?" No. The question is, "Does the system SUPPORT this?"
And the answer, in a rules absent system, is no, it does not support it in any way. The reaction is 100% dependent on the DM. And, if the DM decides that the reaction is something that the players don't like? Too bad. They can suck it up. Even though no one at the table is enjoying the game because of the DM's decision, it doesn't matter. It's all on the DM's shoulders. So, when the DM decides something that the table enjoys (note, enjoy, not like - that's a different thing) then that is a great DM who everyone applauds. But, when the DM decides something that the table doesn't enjoy, it is equally the fault/responsibility of the DM.
Whereas in a system where these reactions are supported by the mechanics, there is a framework in place which determines that reaction. SInce we're all choosing to play this system, presumably we like this framework. Note, we could also, just as easily, choose to ignore this framework in this situation in favor of something the table likes better. No problems. But, with the framework there, you have a starting place to work from. Without that framework, it's all guesswork and, as you say
@Lanefan, trial and error. With error resulting in people at the table having a bad time.
I'd much rather that the system had something in place to support what we're trying to do. Claiming that it's better to have no mechanical support means that you are presuming that the DM will make the correct ruling every single time, regardless of how experienced that DM is.