User blog comment:Njalm2/The Beauty of Zanjutsu/@comment-136273-20150410194427/@comment-3403804-20150410203500

Quite. But seeing as this is mastery of your spirit, it wouldn't be difficult for a Zanjutsu Specialist to use just as much power as they felt was necessary. Their self control in regards to this would probably make it so that they wouldn't need to restrain themselves, as much as they would need to have an intent.

There's of course nothing preventing you from using Shikai and Bankai if you want to, perhaps they might simply find it more familiar and handy that way. Even if they don't actually need to control their power in that particular manner.