Wrapping the thumb allows you to have more control up high on a person's hand. It also helps to protect your thumb while performing a toproll. If you grip someone unwrapped and apply posting pressure (the high back pressure with the front wrist lock that you were referring to) you will readily lose your thumb in their grip, and it feels a bit awkward. If you cap, you can touch their first knuckle joint of their index finger which aids in control and helps to prevent the awkward losing of your thumb. Mid move, your thumb is subjected to a lot of twisting; your wrap also protects you there.
I also just thought that it may have qualities similar to a hook grip in lifting. It may aid your control of the opponent's hand up top. However, it certainly weakens your control on the lower part of their hand. Don't be discouraged by that statement though. Capping is very common and is a strong move.
Recommended for high hook: yes
You were indeed training a high hook with me. I think there is strong evidence that it is the most powerful move in armwrestling. You seem to view a high hook as a variation of a hook. This is not the case; it is more of a toproll than a hook. Don't become to focused on definitions of moves. Even among toprollers there is a great amount of variation. Anthony Snook: great toproller. Sonny Larson: great toproller. Cobra Rhodes: great toproller. Travis Bagent: great toproller. All four have different styles of toprolling.
Snook uses a fisherman dropped wrist toproll. Sonny uses that high hookish, backpressure style toproll. Cobra uses a hand pronation style toproll. Travis uses a posting style toproll. All different. All effective. You can find several examples of each type. Roger Nowatske=Snook. Dave Patton=Travis for example. Engin Terzi is a good toproller to watch (similar to Sonny).
If it feels comfortable to you then use it! Don't stress over the variety of moves yet. Just pick one or two (ie toproll style and maybe hook or press) and work those without focusing on the nitty gritty yet.
That said, the main reason that high hooks don't get described as toprolls is because they don't seem to break the hand back. You apply backpressure and wrist flexion with a pronator isometric which leaves your opponent in a neutral wrist position and supinated (ie palm up...a weak position). Their wrist isn't usually broken back leading to the belief that they weren't toprolled...but the definition is a bit meaningless...if you force them to the position described above, you have basically won their wrist.
There are several pullers that make use of a high hook style toproll. We should have some armwrestling practice after the grip tournament March 9th!