Hmm… I believe yuri is still used even on fast Jonkara. There might be a miscommunication of how much vibrato we are talking about.
Take a look at this - http://www.youtube.com/watch?feature=player_detailpage&v=EP0l52jMSD4#t=120s
As Asano Sho strikes the ichi-no-ito, he’s putting in light vibrato by pressing the string about 1cm BELOW the intended position, and then sliding it up to said position. (This is all done in the period of 1/4 second) Because it’s a light amount of yuri, the accuracy is still evident.
I would say most all professional players do this on the ichi-no-ito, as that little bit of yuri adds more “life” to the sound than just holding the string and thwacking away on it.
Vibrato and slides are all fine. All the shamisen “legends” (like Chikuzan or Gunpachiro) sounded different from each other, because they went for what appealed to them. In the same spirit, there’s nothing wrong with putting in more vibrato if you like the sound of it. The critical part is intention. It’s fine to get wild and slide all over the sao, as long as you show you are doing it with control and intent. In taikai, wide vibrato (from position 6~9, for example) is done to show “controlled frenzy.” When executed cleanly and with intent, it yields big points. Without control, it loses points.
In summary: Like anything in life, what you actually do is not really important. It’s the attention, control and intent that matters.