Every adult should have an LTC - more correctly, no adult should need an LTC...
That said, if you have one in the house, every adult should get one. For all the reasons discussed in this thread - it is a legal hassle waiting to happen and you are a felon if you hand over your keys/combos to a non-LTC'er in your house...
Exactly, you are correct on all points.
If you live in a household in Mass. and you have guns, everyone else who is of age to be licensed should be licensed (15 for FID, 21 for LTC). This will remove any gray area if someone accidentally posesses guns/ammo/mags/ a frigging empty shell casing, or anything else. As others have said, sickness, injury, loss of license, or any other number of situations could come up where having multiple licensed persons would be a good idea.
Secondly, I tell anyone who even thinks they might want to own or carry a gun to get licensed (if required) ASAP, simply because there might come a time when they need a gun faster than the licensing process allows (Sylvester Lindsey got hosed by Mass. gun laws because of this, here's a link to his case http://masscases.com/cases/sjc/396/396mass840.html).