Keep in mind that the VP determines his own involvement with regards to running the Senate. The Constitution provides that the VP do the job and the PPT's power is entirely derived and delegated from the VP.
The problem isn't as much about ability, as it is willingness to engage and communicate with the PPT to stay involved. Kalwejt typically does more than most VP's, atleast judging by his past times holding that position.
Well, personally, I would say that Tmth was very active in his job as VP last time, but his role was still very limited in Senate affairs and the PPT usually has complete autonomy in running the Senate. Designating the full powers of a Senator to the VP only adds another person to the legislative process and makes the position itself more important to the game.
Whether or not the PPT has complete autonomy is a decision that the VP makes himself. As I said, the VP could render the PPT a total spectator if he wanted to do so. All of the PPT's power is delegated from that of the VP. Which reminds me, Kalwejt has yet to post a public delegation of authority to me. 
When has the VP ever taken control of the Senate himself, though? At this point, it's seem to become tradition to delegate all the authority to the PPT, and it would just be logical to make that normal routine.
I can't give you an exact time period, but there was a while a couple years ago when VP Bacon King served as PPT for a while, and actually did a very good job. I can't remember the reasoning for why it occurred, even though I was in the Senate at the time.
