Not sure about this. 20% time put some boundaries, and marks that you should be doing something different. It can be typically abused (hey, I'm supposed to have 20% for "other projects", but I have a deadline, so I'll use it for the "regular work"), but at least there is a line in the sand saying: "This is clearly time for OTHER STUFF". Your manager can say to you "hey, it's friday so you should be leaving this alone and do alternative projects"
100% sounds great, but does not define anything, so it's extremely easy to fall into "I have a lot of work, I can move to other stuff when the workload goes down". Of course, workload never goes down...
(Author here.) I agree that 100% time does not define anything. That is definitional. ;-) I'm "the boss" of the business, so I'm pretty well in control of what gets done and what doesn't.
The whole thing requires a great deal of self-control and psychological awareness…though I'd suggest that without that, the business that makes the whole thing possible in the first place probably wouldn't exist.
100% sounds great, but does not define anything, so it's extremely easy to fall into "I have a lot of work, I can move to other stuff when the workload goes down". Of course, workload never goes down...