Robert Frost hates unstructured play

Actually, the relevant quote is “I’d sooner write free verse as play tennis with the net down.”
In the most recent episode (15) of their Magic podcast DeckConstruct, hosts Alex and Dan go to a local Magic scene and ask people what they think of casual play, as well as how they’d define it. The consensus understanding of “casual” is “not tournament play,” as embodied in the phrase “It doesn’t matter if you win or lose.”
There is, of course, nothing wrong with this. If every game were about qualifying for a Pro Tour and a $25 entry fee, I’d be a pretty grumpy camper.
But even in casual play, there must be structure. A lot of the interviewees said that they liked how they could bring “any old deck” to a casual game, no problem. But there is a problem, inasmuch as without any particular rules, it’s hard to say what you can bring. Or, to put it another way, “Sure I can make a deck that sucks, but how do I make sure mine sucks as much as yours?”
In competitive Magic, this is easy. Format? Standard. What can I play? Anything in Standard. Put in the best cards, optimize your deck, try to win. Everyone’s on the same page.
In default casual, it’s unclear. This is social contract territory, because the “rule” really is “try to win, sort of, but not too hard.” If you go into the “tournament practice” area in Magic Online, you will occasionally run into people who haven’t made the best possible deck. You will run over them, and that’s okay. If you go into the “casual play” area, it’s entirely rockier. Hit someone up with a Stone Rain and you may find them complaining publicly that you suck for playing land destruction. Or perhaps not. Who knows? There are no hard-and-fast rules, and this kind of casual play often amounts to “whatever I don’t feel put off by.”
That’s a vague, vague rule to follow.
My preference is for structured play. Rather than the fuzzy implied social contract, set an actual contract. Play Standard. Play Extended. Play Highlander, Pauper, or anything else with defined rules. I want to be on the same page with my friends, whatever that page happens to be. The fundamental problem with the implied contract is that you’re trying to play suboptimally, and there’s no good way for everyone to accurately be equally bad. Someone may well accidentally bring an overly good card to the dance, and then they just keep winning over and over again, which isn’t fun for anyone.
Back when I played Mechwarrior a great deal, we had an explicit agreement across the tournament players to play “faction pure” forces (that is, forces derived all from one faction within the game, a situation not required by the game rules). We did this because pure forces looked better, and because pure forces came with inherent strengths and weakness that mixed-faction forces smoothed out. Had we not formalized this, the one person who didn’t care as much and showed up with a mixed force might well have walked all over the others — whether they really wanted to or not!
In gaming, as in the rest of life, I like my social contracts to be explicit. When everyone’s on the same page, it’s just that much easier to have a good time.