Subscription billing for a B2B SaaS

It's a question of how much of the work do you want to do yourself. There are full service solutions, which are all-inclusive, making it so there is little development work needed, just about every feature you'd need now/later is available, including global currency, language, and tax support. Then there are plenty of more basic solutions, which may be a good fit for you if you're open to doing a lot of the development work yourself or if you don't need most of the features offered by a full service subscription management and e-commerce solution.

There is also the issue of advanced dunning management that both chargify and recurly include.

Infact, chargify has a whole email management process for late payments. I think the value of subscription billing providers isn't just in the ease of accepting payments and flexibility of switch gateways, but their expertise in billing.

Actually, Spreedly just wrote a blog post about this: http://blog.spreedly.com/2012/3/27/spreedly-core-use-cases-single-gateway-why-do-i-need-spreedly

Boiled down a bit, it's because you want the freedom to switch payment gateways (and merchant accounts, which many payment gateways may bundle in). If you're not using a subscription management service (or storing the credit card data yourself (shudder)), you'd have to get every one of your customers to reauthorize their subscription with whichever service you switch to.