Lighting – put a LED tube in a high frequency ballast fixture without removing the ballast

First, that fixture looks permanently hardwired. If so, you'll want to add a ballast disconnect. Code requires it, unless you argue that the fixture is now ballastless. But it's so darn convenient that I add them immediately upon starting work, and then, turn the power back on!

Cutting the wires off at the ballast is someting I only do to ballasts which are dead. It's already been wirenutted from the previous ballast swap, I would just do it at the wire nuts.

Generally, ballast instructions are generic and simplified, try to come up with a universal instruction and you can see where it's hard. Potential reasons for the "leave the ballast" instruction include:

• different ballasts require disposal in different ways, and it's complicated and they don't want to open that can of worms
• in particular, they don't want to scare the hell out of people with talk of PCBs
• it may greatly reduce the weight of the fixture, with unknown consequences
• it will shift the fixture's balance point, relevant to chain-hung fixtures
• removing the ballast may leave holes on the fixture uncovered, which Code would disallow
• in some bad installations, the screws holding the ballast also help hold the fixture to the ceiling.

Direct wire. Worst case you have to change a lampholder.

Plug-n-play doesn't allow a ballast, it requires a ballast! And of a specific type - if the fixture has the wrong type, you would have the ignominious task of buying a new ballast and also possibly change lampholders too. It's one more thing to maintain.