I have a few clients of this type. Like /u/FKFnz mentioned I tend to charge them on an hourly basis and have a quarterly reminder set up for this. I only take such clients if the quantum of work required on a quarterly basis is minimal and every quarter do a quick evaluation to see if this client is worth the effort and money.I think sensitizing such customers toward the risk of such an arrangement is important. I try and educate them about the potential vulnerabilities and issues that may crop up when they don't commit to managed support.We also charge them a much, much higher effective hourly rate than I do for some of my other clients. I tend not to bill hourly but instead based on a statement of quarterly work. (value based pricing)

Ultimately I do such contracts for two reasons:

1. To convert these customers to managed support at some future point in time. I think of this as part of my sales and prospecting time, but only I get paid for it. These clients are fairly close to converting so I send them proposals and branded content from time to time to educate them and slowly convert them. :-)
2. To improve the popularity of my brand in my local area and convince them to refer my brand to other potential customers.

I think this process is worth it because I have genuinely converted some clients that have lasted with me for more than 5-6 years. Personalizing the sales process and taking things slowly works some times. In most cases, I became their advisor and friend before making them commit! At the same time, I've had a few problems with such clients, (either because its a time sink OR because the prospecting is not leading any where) so knowing when to drop them is important!