I totally understand what you mean about the pillion being part of the bike/rider combination,if you have a competent and confident pillion on the bike the handling in not compromised to any degree and beside a reduction of clearance you can take the corners at good pace , in saying this I am not a boy racer type of rider so do not have to slow down from my normal riding pace with the Princess on the bike,
One thing I am the worlds worse pillion ,I absolutely hate being at the Mercy of another rider