You can connect, but your bags can't — UA and AA's latest service cut

No one doubts that when possible, it’s best to have an entire connecting trip on one ticket. On the other hand, sometimes that just isn’t possible. Or, it’s a lot more expensive.
For examples, anyone using a free ticket with mileage and then connecting on needs to have two tickets.
And, while within the U.S, often multiple fares might be allowed on one ticket, this isn’t always true. Sometimes, especially for travel overseas, there are often fares to a gateway city, say, London, Heathrow or Frankfurt, and then a highly discounted and restricted intra-Europe fare, which is not combinable.
As a travel agent, I’ll warn travelers that they may face issues with two tickets. Sometimes, however, it’s still the best solution. Be aware: There are issues. One of them can be checked baggage.
In general, while such baggage acceptance is at the agent’s discretion, Delta Airlines in 2013 announced they would stop checking bags to the final destination for passengers holding two separate tickets. Then, they backed down after an uproar.
Then last October, American Airlines announced they would only check through baggage for customers traveling on separate tickets if the separate ticket is on American, American Eagle, US Airways, US Express or a OneWorld Partner airline.
On March 1st, United Airlines is quietly about to follow suit. The airline rule says, “Bags will only be checked to the origin and destination on a single or conjunctive ticket.” (A conjunctive ticket dates from the old days of paper tickets, when a ticket could only have four flight coupons. Anything longer required two, or more, tickets. Now, with electronic tickets, it just requires two or more ticket numbers.)
The rule adds that customers traveling on another ticket with other airline space must collect their bags and recheck with the other carrier. Plus, presumably, pay another fee. Even worse, the rechecking process means the need to exit security and go back through the entire TSA process.
The new rule, however, doesn’t say what happens, as with a mileage ticket combined with a paid ticket, if someone is connecting between UNITED flights. There will be two exceptions, when a traveler is connecting to a Star Alliance carrier. Which means most but not all of United’s partner carriers.
Curiously enough, while airlines like Air India and Lot Polish Airlines are “Star Alliance,” commuter plane partners like Cape Air and Island Air are not. And, connecting short hops, say a Cape Air flight from Boston to Martha’s Vineyard, or an Island Air flight from Honolulu to Kauai, seem to me to be amongst the likeliest times when travelers might need two tickets.
Travelers who are Premier Gold level or above with United will also be exempt from the new rule, and can go on checking their bags as before. Such elite members aren’t paying baggage fees with United anyway, so they’ll save both time and money.
Now, airlines like Southwest have never had interline agreements so have never checked bags on other airline. But in my experience, legacy carriers have been much more flexible.
With United following American, however, it seems likely that this will become industry norm.
The best thing about this new policy is that it won’t affect that many people. For now. Although you have to wonder, if there is little pushback, will the next step be not to check bags for passengers on flights that connect to another airline at all?