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I just noticed, reading the Dungeons & Dragons 5E DMG that there appears to be a difference between the dancing sword from previous editions. Unlike previous editions, there doesn't seem to be any "after dancing, the weapon can’t dance (attack on its own) again for 4 rounds" clause in 5E. (This particular quote came from the 3.5 SRD)

However, even in 5E there's still this bit in the dancing sword's description:

After the hovering sword attacks for the fourth time, it flies up to
30 feet and tries to return to your hand.

But:

1. Is there any reason you can't just use a bonus action to toss this
   magic sword into the air and speak the command word once more,
   effectively having the sword dance every round all day?


2. In what way is the "return to hand after four rounds" a limitation
   at all in practice, assuming there are no limits to dancing? There
   seems to be zero difference between using your bonus action to
   have it dance, and using your bonus action keep it dancing.

The sword might not make it back

So first thing is that the blade doesn't always make it back:

If you have no hand free, it falls to the ground at your feet. If the sword has no unobstructed path to you, it moves as close to you as it can and then falls to the ground. It also ceases to hover if you grasp it or move more than 30 feet away from it.

If this happens, you are obviously going to need to move to the weapon and pick it up if you can. However, if you already used your bonus action to have the weapon move and attack (or for anything else) then you will not be able to reactivate it that turn.

Remember you must have the weapon in hand to cause it to activate it:

You can use a bonus action to toss this magic sword into the air and speak the command word.

So there is a difference, you cannot use a bonus action to keep the blade dancing only to start it dancing again.

Assuming the blade returns safely every time there's no reason you can't have it hovering almost all the time

After returning to you, you can use your bonus action next turn (you've already used it this turn to have it move/attack). But other than that there's nothing restricting you from using the feature again next turn. With smart usage, you could be able to have it hovering almost every round if you wanted.

Note that it's clear that the designers did not want a limit put on it. If they did, they would have kept the wording from previous editions. They just have had their own reasons for doing so. Perhaps the use of the bonus action was deemed cost enough. For some characters, this is a very valuable resource. However, for others, it's not at all.

Whatever the reason, as long as you can get the weapon back into your hand, there's really no limit on you using it every round.

Different editions make a lot of changes

There are many changes made with each new edition of D&D. Honestly it's best to think of each version of D&D as a completely different game with different rules in a lot of ways. Given that, I'd honestly be more surprised if the item was kept exactly the same from 3.5 through 4e and into 5e.

There's a lot of reasons the change could have been made, but the important part is to realize that changes were clearly made and it no longer functions as it did in 3.5