Depersonalization in BDSM (not to be confused with the psychological disorder) is an interesting fetish.
I've honestly never met a Domme that really enjoyed this with any subs she cared about. That doesn't seem to stop many men from fantasizing about. I'm guessing there are women out there who would rather enjoy a stable of depersonalized slaves serving their every whim, but chances are this too is probably relegated more to the fantasy realm.
What is it about depersonalization that makes it an object of fantasy?
From a sub's standpoint I can see it as a rather extreme form of submission where you are treated with indifference, often as a nameless, faceless servant or slave that can expect no pleasure from his Mistress. Denial of pleasure is often that great, deep, "I don't understand this" fetish fantasy. That's not to say that it doesn't turn me on a bit, but I still don't really get it since I know I wouldn't enjoy it if it were to happen for longer than a few days. For some reason, there's a bit of a charge for many male subs at the thought of being 100% expendable and replaceable.
From a Domme's standpoint I can see a few potential appeals. Having a nameless, faceless sub to be tortured at any level of intensity guilt-free, alleviates some of the mental burden of wanting to perform acts of significant cruelty that she is unwilling to perform on a sub she cares about. The fantasy of having a hoard of nameless, faceless subs that exist only to serve her can give an elevation to almost Goddess-like status.
So how often does this actually happen? From my experiences, not often, but it can be used as a role-play scenario during a play session to a mild degree of success. I can't say I've met anyone with a 24-7 D/s relationship who practices this regularly.