I disagree with this for two reasons:
1) What if the coach is able to secure a last-minute tourney slot, but then the team can't accept because of key players guesting on other teams and not telling the coach? Can you imagine the conversation like: "Yes, we agreed weeks ago to sub on another team, but we didn't think you had any reason to know."
2) Imagine a pitcher subbing with a different org, then knocks teams from your org out of the tournament? We had that exact thing happen when DD was playing 12U. DD and two teammates subbed with a lower level team, and ended up knocking a team from our parent org out in the semi-finals. "Our" pitcher was the winning pitcher in that game, beating our org team. That caused a massive "poop storm" between the coaches and parents at the game, and after the fact as well. The next week, the org president passed a rule that you can only sub with teams within our org, and if those opportunities don't exist, then he has to personally approve them.
An unusual situation. Guesting arrangements that I've seen are typically made days, or even hours, in advance...not WEEKS.
Regardless, getting notice of a last-minute tourney slot is MUCH different than a last-minute practice. When I've agreed to guest my kid more than a day or two in advance, I've done it with the provision that if her team scheduled a tournament, the guesting agreement would be void. Not a hard situation to avoid for a player who is committed to her team first rather than random playing opportunities. I'd still not allow a coach to have the final say as to whether she played on a particular weekend.