I understand the marketing, but I can't help but feel faintly irritated that I'm already paying for a Skype subscription. Should I cancel it and get my free month and then renew it again? ...
It's the same as all other companies. "Switch to our Halifax, and get £100 free. Offer only available to new customers". It's rare to see a company that rewards existing customers instead of disregarding them and focusing on the new ones.
Pleasingly some mobile networks in the UK are doing this, but that's only because they have an incentive to retain customers. I don't know of a good competitor to Skype. Google Voice, perhaps, except it lacks subscriptions and a nice native iOS app (to my knowledge).