First time poster, long time reader. It seems Thibs is a big proponent of specialisation routines, and I have always been interested in a lot of the suggestions and outlines he discusses in some of his articles.
I think you hit the nail on the head with your comments around frequency playing a big part of how these routines would work for an individual. As an example, I work on a 6 day split: Chest - Back - Quads - Shoulders - Arms - Hammies. When I do any specialisation work I combine the two leg days and mix in the second bodypart day somewhere that fits well. As you mentioned also, I will run this for about 4 weeks. I don't continue with other bodyparts though, I roll straight back into my 6 day split (unless there is another glaring weakness I want to focus on).
Another example I am currently using is a chest specialisation. This is currently my biggest weakness, mainly because of my dominant shoulders. So I maintain my stand alone chest day, but drop the volume on shoulder day and add in 2-3 chest exercises. This way my shoulders are pre exhausted and don't take over too much on the chest exercises..
Long story short, I really think they have their place in training, particularly around weaker body parts. However, I try to maintain intensity and volume across other body parts also.