What warnings do you heed?
The feed validator knows things that are errors, and things that are warnings. It knows that the RSS spec is silent about whether or not you can use dangerous markup like on* Javascript events and objects with nasty payloads, but that any sensible and conservative aggregator author will strip them out of your feed before sticking it into their user’s browser, and so it will tell you in /docs/warning/ContainsObject.html (in a little too strong of language, since in fact you only need to consider whether your use is safe, and whether your content will still work and make sense if the offending tags and attributes are stripped). However, since it currently only knows one way to get your attention, it will tell you that by saying “You, Sirrah, have an Invalid feed!”
So, my question: what warnings, where a computer tells you “you’re good, but…” do you actually heed? If the feed validator told you “Congratulations! Your feed is valid $spec. However, here’s something you need to consider about whether it will work the way you expect.”, would you listen?
That’s long been a debate about the feed validator, and as you correctly point out, it is merely a UI issue (although an important one). Internally the validator knows which things are errors and which are warnings, it just reports both equally.
Constructive suggestions are always welcome. Criticism of the form ”that’s not explicitly mentioned in the spec in big red letters, so therefore YOU MUST BE STEALING RSS” are less likely to be taken seriously. Being a validator developer gives you a hair-trigger bozo bit.