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.
That would work, you could do it manually or sync with the app. Right now just supports Google Calendar sync, but would make sense to allow for any .ics file. Hoping to add that