IKillU69X posted a thread about this already, but I thought I'd elaborate on the topic a bit more.

As it is, if you pick up a spawner with silk touch, the spawner will still say whatever mob type it is while it's in your inventory. Once you place it, though, it reverts back to a pig spawner.

Previously, this was mostly just a mild inconvenience. Now that /spawner has a 20 second cooldown, this can get rather tedious to move farms. If I want to pickup all of my spawners and move them somewhere else, it requires a total of 16 minutes to revert everything back to its original state.

Pros:

• Removes the need for mods to get involved when moving/selling guardian spawners (also useful for other spawners that can only be changed with spawn eggs, like parrots and horses)
• Allows players to directly sell/give certain mob types to players without access to /spawner
• Lets you move spawners without need to change them back after placing them again (especially useful now that /spawner has a 20 second cooldown)
• Less confusing for players, who may be confused why their spawner changes to a pig spawner once placed, instead of being what the spawner says it is in their inventory

Cons:

• Will make selling spawners slightly more tedious since it'll be harder to convert large amounts of spawners to pig spawners

 

I've never been entirely sure, I've always just rolled with it. I know the /spawner command cooldown was to help with performance, as well as avoid an exploit you could do by spamming the command. Not sure if there's an official reason why spawners don't retain their mob type, though. Can't see how it'd influence performance.