Interestingly, sandbox isn't all that great for learning the ropes. Sure it's nice to see how the ship it laid out, but you can get that information in the first 30 seconds of a match. Pretty sure practice mode is mostly used by the higher level players checking out new builds and testing things to make sure it's all in order before going into live combat.
For the teaching achievement, I might be able to keep a player in there 30 minutes or so trying to get them to try everything out until they feel comfortable. Then waiting for them to ask the 'Oh, do you have any tips?' before I start giving every tip and hint I know (players have written novels about tips and tricks for this game). Eventually they'll get bored of the lecture and head out.
Being able to have multiple ships in a practice match might help a lot with players getting bored instantly and actually turn into a better teaching tool. However, you usually have to have multiple students and teachers for that to work well. Being on the opposite ship of your pupil makes it harder to explain things, since you can't see exactly what they're doing and you're forcing them to pilot instead of trying out their gunner or engineer skills.
Back on topic I feel that not having it be private could lead to some frustration with players having their builds seen before they're ready or get caught testing some weird function. Not to mention the practice mode commands possibly ruining a good practice match if used by a troll.
