it's an interesting point, could you please go into a bit more detail on how you structure the retainer (certain amount of hours per month)? How many you can stack and so on?
i managed to change a multi year engagement that was billed hourly or by day into a fixed retainer. not knowing what to charge i simply asked the client to tell me what it was worth to him. he chose the amount that i earned with him in the past year. i accepted that, figuring that i would renegotiate if the work got more, or simply not be available for more work than what the retainer was worth to me. what happened in practice was that the work got less. this is circumstantial of course. it happened mainly because we both moved cities and were no longer able to work face-to-face. arguably that made my work more efficient as i no longer had to spend time to travel to the office and i could parallelize multiple things that i was working on. my effective hourly rate ended up being somewhere around $300-500. you can't charge that when billing hourly. i happily would have worked two or three times as much without asking for more.
I've had concurrent retainers, but I'd avoid over-stacking them. Retainers are a liability, you could have multiple clients ask for hours simultaneously, and you'll suddenly be very busy. Weekly retainers are better than monthly because they smooth out the hours.
Personally, I divide my availability into contracts and the more concurrent contracts I have, the more I pad for overhead. So one 40 hr/wk contract vs three 10 hr/wk contracts.
It's possible to stack more, as often retainers aren't fully used, but I think it risks the relationship with clients, which is worth more than over scheduling yourself.
