Personally I need and want the emotional connection. I've tried casual play and it is unfulfilling for me similarly to how casual sex has always been unfulfilling for me. Partly I think the development of trust for me stems from the emotional bond. I need to care about the other person before I can drop my guard and allow myself to feel submissive to them. I need to feel they care for me on some level before I feel safe to be as vulnerable as I believe necessary to access my submissive nature to any significant degree.
I am an affectionate person in general and so once an emotional bond starts to form with a dominant and I can begin to feel genuine affection for them when we match up well. When that happens those feelings only tend to deepen as the relationship goes forward. I don't find it at all threatening if my dominant does not return my feelings in the same degree. I just need to feel assurance that she does feel affection for me on some level so that I can feel that I am accepted for who I am and can feel comfortable dropping my guard and allowing her to see me for who I am without any pretenses.
As far as the poly issue, I can and actually have dealt with that without any problems. I'm not in that sort of circumstance at present but if my current Mistress were to develop feelings for another, as long as she made me feel I was still important to her within the context of our relationship and that there was no change as far as how she felt about me, the simple fact of her being involved with another person or even more than one would neither dampen the tender feelings I have for her nor cause me to rethink my willingness to submit to her.