giving my opinion here, I would think that, being dominent over another, it comes down to "control".

On 1 level, I would wish to maintain complete control over my sub. Not, if they were in touch with a previous Dom, fall into being submissive to the prior one out of habit. That takes some of my control away, allowing another to influence, for good or ill, the sub I am trying to shape.

On another level, by denying such a contact, it might bring about negative feelings on my subby's part, for taking something that deep within them, away from them. Seems to me, that a good Dom(me) wants to use and enhance and thereby control the subs feelings and thoughts, and if that means allowing them to learn/ build/ remember their past and how they got here, it would just be one more tool for me to accept and learn to use to my advantage.

I know, ambiguous response, but, I guess what I must reiterate is the "control" feature. If I can control my sub through everything and everyone they have interactions with, then I am a success. Perhaps it is a nice growing feature in a relationship, to test it out this way once in a while....