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.