Take customers acquired in, say, July of 2011. For the next twenty-seven months, calculate the probability of the customer purchasing in that month.
If your resulting graph looks like this graph, then birthdays and anniversaries are important to your business.
When a business has a customer base that purchases fewer than two times a year, the graph frequently looks like this.
In months 1-4, the customer is highly responsive, adding items to complement a first order. Then the customer goes dormant. In months 11-13, the customer "reappears". Maybe the customer buys gifts on an annual basis - in that case, you're bound to see the customer reappear in months 11-13. And again in months 23-25.
If you are in the personalization camp, then this is a graph you have to run. Get a feel whether birthday/anniversary events are important in your business.