Hey, cool looking product! Why do I want this instead of just adding birthdays to my existing calendar and setting reminders there?
Adding birthdays to a calendar would save you the $9/yr and may be preferable for some users. I just prefer text reminders—they are hard to miss. For me it's the simplest, least intrusive way to remember. And I like being able to add new friends via text as well. I'd rather wake up to a text that says "Happy birthday to ____!" than check my calendar to see a friend's birthday next to random events.
As a middle ground, I wonder about adding everyone you care about to the calendar, hiding that calendar in your default view, then using it as a data source for the text reminders.