SaaS is the "S" in SPLA

I've found that relatively few people know about the Service Provider License Agreement, which was built for hosters and companies which need monthly, usage-based licensing.  From the program description:

The Services Provider Licensing Program enables services providers to license Microsoft® products on a monthly basis. The program is designed to emulate how service providers do business - providing services on a monthly fee to their end-customers. A sample list of products available to service providers in this program is provided below.

So it's a pay-per-use model that works in arrears - hosters provide a report of the license units used that month and pay after they're done.  SPLA incorporates Software Assurance so there is only one license and one price to pay for usage.  There is a version specifically for ISVs which you should look at if you're currently a SaaS ISV using the Microsoft perpetual license model.

Two good examples of why this is useful:

1. Cashflow management: under the perpetual license you would need to buy the software up front and amortize the cost over a multi-year period.  SaaS is a cashflow-focused business model with little room for capital expenditures that need to be rationalized later (as seen by the standard practice of leasing hardware rather than buying)
2. Seasonal volumes: if usage goes up one month, and down the next, there's no need to buy additional perpetual licenses, guess what the necessary capacity will be, and suffer for guessing too high (if you're in Operations at a SaaS company, you know how challenging this can be - as they say, "it's hard enough to predict the past, let alone the future").  Instead, SPLA users report & pay month to month, so payments go down when usage goes down.

The team that came up with SPLA, led by Pascal Martin, has partnered with Rackspace and other hosters so that SaaS ISVs can transfer their existing operations to an MSP.  They are working hard on the next version of the ISV-focused hosting solution - more news on this in the near future.