DD's org discourages playing with other teams, but I can see a future once she gets to 16U (she's 14 now) where arranging to play with another team in a showcase where you know a specific coach is going to be will be important. To have that situation be the first time they've ever been a guest player would be a disservice, I think.
Also, from the college coach's point of view, I would think that they would be impressed if a prospective player emailed and said, "I see you'll be at XYZ Showcase, I have made arrangements to play with the Golden Poodles if you'd like the chance to see me play." That shows serious interest as well as initiative.
Excellent point! When DD was younger, I also encouraged her to guest play for other teams when possible. That way she isn't always playing for dad... I wanted her to have the opportunity to play for other coaches and work alongside different teammates. Honestly, I think those experiences had as much to do with her success as playing for her own team.