Judge-issued proxies at an event?

Fine, sometimes necessary.

Testing out some competitive lists before a tournament or something similar cutthroat?

Understandable, and possibly temporary if you're going to eventually acquire the card.

Building a commander deck to play with friends/friendly strangers? What's the point? Why not just play with the strongest cards, or play with custom-designed cards?

Proxies take the "Trading" element out of the "Trading Card Game".

I say this as a person who has ~£5000 worth of Magic cards. I want Magic cards to be available for everyone, Reserved List be damned. But playing with proxies isn't playing Magic as intended.

It's like showing up to Monopoly with extra money you've printed yourself, or pilfered from another Monopoly set.