Since its release the Murloc Costume vanity item has always broken upon receiving damage or casting a damaging ability. Back in 2007 blizz stated the following:
“While we are very happy for the people who had the chance to attend BlizzCon and pick up a code for the Murloc suit. We chose not to allow people to fight/cast spells with this on, similar to the Hallow’s End wand transformations, as we don’t want the whole game to eventually have nobody that looks like races that are in it. We also didn’t feel like having this suit on while fighting epic battles would fit the feel of the game.”
This response is so incredibly outdated. We have several ways that people can copy the costume and use it in combat i.e. mirror and also you sell items that change your appearance for an hour without combat breaking the effect.
The murloc costume is also a very niche item and also expensive to buy today. You aren’t going to see an entire raid with everyone using one of these; you will be lucky to see just one person use it every now and again…
It’s not going to hurt anyone or cause issues to enable this. Even the animation argument is not relevant as we have 100s of transformative toys that are limited and no one complains about those.
I’ve had mine since 2008 and used to use it regularly when social areas were populated on my server, these days not so much… Being able to use it in pve content will make a world of difference 
I’m sure everyone that owns the costume would feel the same way.
Many thanks!
