Yes, but (a) Bilbo had a bond with Gandalf, and (b) a compulsion to give hospitality (like a fate aspect the DM could tap) and follow his promise, and (c) the author can make Bilbo be reluctant but go along with multiple editing passes. Players often don't like it if your DM does multiple editing passes over your decisions.I seem to recall that Bilbo Baggins didn't want to adventure with a bunch of obnoxious dwarves. But anyway, if the PC doesn't have a reason to stick with the party, the GM should assist by providing an incentive for that PC to participate. It takes two to tango.
The goal is to make marginal players good, and poor players marginal.It is. But from the perspective virtue ethics compared to duty-based or situational ethics. I believe that proposing a list of specific dos and don'ts is ineffective. Instead, I look for players who naturally exhibit the virtues that contribute to my subjective definition of good gameplay (intelligence, humor, creativity, individualism, confidence, well-spokeness (get the joke?), reliability, honesty, etc.)
Having clear expectations helps people do that.
That may not be your goal. You might not consider that worth your bother. You have a supply of players who are awesome and everything is perfect and an unerring ability to find and filter out dross players.
That is fine. But meanwhile, for the rest of us, being able to take someone who is a marginal player, add a rule or two and get a good player out of it, or a good player, and encourage them to be great, has high value.
To me, this means not having a huge long list of rules. Having a few nice rules that nudge players into being more enjoyable would be my goal.
If you need 50 rules to be an acceptable PC, you just playing whack-a-mole with every poor DM-Player relationship you have had. And that sucks.
Figuring out minimal such rules/responsibilities is the thing, to me.