I believe you are referring to a swivel wheel trailer. They are actually a nice idea. They are much better than hanging a platform off the back as around 2/3 of the weight is supported by the wheels not the 5er frame. Also, you can back them up as they stay in line with the 5er.
It would be a slight cost but no reason, you couldn't add an adapter to pull directly behind the truck if you just wanted to move the bikes.
A toy hauler is nicer for the bikes but unless you go really big, they majorly eat into the interior space.
As far as storage on the lot, assuming they aren't very tall, friends park theirs under the 5er overhang. They don't bother but you could add canvas sides that would keep them completely out of sight...or some of the swivel wheel trailers I've seen have enclosures.