Boeing's Starliner is more expensive than Crew Dragon and seems not to offer any significant advantages. Is there still value to NASA or other parties in the future to having a choice between the two?
Commercial Crew awarded two providers for dissimilar redundancy.
This is exactly why NASA decided to select two partners in the commercial crew effort. Having dissimilar redundancy is key in NASA’s approach to maintaining a crew and cargo aboard the space station and to keeping our commitments to international partners. It also allows our private industry partners to focus on crew safety rather than schedule. The safety of our commercial crew team always will remain as our top priority.
This is especially important in case SpaceX's Crew Dragon or Russia's Soyuz have an issue. When Soyuz MS-10 had an in-flight abort, NASA's access to space was put in jeopardy. Fortunately, the issue with Soyuz was quickly rectified before the ISS needed to rotate its crew.
Space-X has a completely radical design and testing philosophy. NASA can't ignore it, because it produces results very quickly and costs much less than the traditional model. However, NASA also cannot fully commit to an untested paradigm. More generally, it's smart to diversify your portfolio if you don't know enough to pick the winners.
There's also a certain political advantage to any inefficiency that the traditional model produces: it means more jobs for more Americans; and if the design process is slower, it means those jobs are more permanent. Allowing tax-payers to invest in the process (by working for Boeing, e.g., or owning a convenience store near a Boeing factory) makes it easier to justify legislators' continued support for NASA's programs.