I tend to agree with Papa Sweet's ideals; I don't want to ever be an SB's sole source of monetary, emotional, or sexual support. My rationale may be a bit more pragmatic though.
Exclusivity tends to suggest reliance and dependance on me to ensure her needs are met - all of them - even if she develops needs beyond the boundaries of our arrangement. I don't want to be her 1st (or 2nd, 3rd, or more) call when she has a serious need or emergency. Nor do I want - let alone expect - her to devote all her otherwise free time to me. In addition, I know I will never be willing (or frankly able) to cover all of the expenses needed to support the lifestyle she wants to live through her endeavors from Sugar and "civilian" means (i.e.: a job, parent support, etc.). If I really want an SB to be "on call' for me 24x7 I should be willing to fund all of her needs all of the time (I think that's called a wife.

. Finally, if (let's face it, when) the arrangement ends, I don't want her to face serious issues like losing her car or being evicted due to the loss of my allowance to her.
Having said all that, I always explain before we make an agreement that I will not ask her to be monogamous with me. I explain that I feel I have no right to control her time or activities when we are not together. I only ask that we both ensure we are keeping each other safe when going BCD. Even so, many of my SB's have told me at some point that I am their only SD. Interestingly, I've been told this at the beginning of an arrangement, as well as after we have been dating for a while. Note that I generally do not tell her that I am seeing other SB's, nor do I deny it if asked directly. When I explain that what she does when we are apart is none of my business, they generally get my implication that the reverse is true as well.
Finally, the concept is situational and dynamic. Obviously, my porn star SB's are fucking other guys (for work) and are probably (make that certainly) seeing other SD's (and let's face it escort clients) as well. One of my SB's has a long-term BF. Their relationship is rocky, and she sometimes asks me for relationship advice (which she generally ignores. LOL). Another SB, told me after we had a few dates that she dropped her other SD. She said she just didn't like him and was glad she found me. Predictably, she spends more time and effort communicating with me between dates (pics from her day, sexy pics, notes on family events, etc.) and has basically told her mother that she has a BF who is much older than she is. (It's an Asian family and I'm speculating that age-gap relationships in her culture are not too unusual.)
So where does all that leave me: I do not want to have, and I do not want to ask for exclusivity.
Final note: I always assume my SB is fucking someone else, and I assume that she stays with me PRIMARILY for the money. My test of this theory is simple: Would she still date me if I stopped giving her allowance? Yeah... didn't think so. LOL
Life is good
The Cat