Never seen used in the practice though. One of problems AFAIK - most switch default to dropping jumbo frames. You can enable but if you'll left one unconfigured for some reason you will get hard to diagnose problem. The same with end hosts - they (inside a given L2 domain) should have the same MTU and if you'll left a box with the default MTU you got a problem.
To make jumbo frames easier to use switches should forward them by default and hosts should accept frames large than MTA (but send MTU sized ones).
Jumbo frames are used constantly in practice, just not often on the internet. They're common in all sorts of networks, especially enterprise and storage.
If your switch is configured on defaults and not running a for purpose config, you probably don't need jumbo frames.
To make jumbo frames easier to use switches should forward them by default and hosts should accept frames large than MTA (but send MTU sized ones).