I think that RSS subscribers matter less and less as services like Facebook are becoming not only a bigger traffic driver but a better way to syndicate content. After all, my mom reads uses Facebook but she will never understand how to set-up and maintain an RSS reader.

Having said that, I still think that RSS subscription numbers can be an important metric when negotiating advertising prices, especially if you do a "sponsor" for your feed (not to be confused with using AdSense, meaning, you plug a company with a special post that also gets syndicated to your feed and in return they pay you an amount weekly or monthly). I think it can be a way to track engagement if your readers use Google Reader primarily, but again, I think Facebook is a much, much better tool on that front.