Aloha to UA (and vice versa) seems pretty good these days; AQ and UA both now can "see" each other's e-tickets.

But if you're connecting to/from HA/AQ/WP another mainland carrier, it's useful (and sometimes necessary) to have the e-ticket number for the other carrier.

Depending on schedule, I sometimes have flown UA from the mainland back to HNL and then connected (separate ticket) to HA for an interisland flight. Sometimes when checking in with UA (or especially UAX) I have to tell them how to interline the bags and show them the HA ticket number and then check the bag tags very carefully!