No matter how roomy the trailer, the truck is a great place for overflow storage, keeping the trailer as uncluttered as possible. The cap gives much more versatile storage.
Also, if you ever add a generator, the truck bed with a cap is a great place for it, keeping it out of the weather and more secure.
Finally I have a bed in the truck and will often drop the trailer and head off to somewhere wonderful which I couldn't visit with trailer in tow.
If none of that applies to you, the bed cover has a different look, and if that makes you happy, that has value too.