[EwanToo]

There's 2 main reasons. 1, if the AZ that your single instance is running in dies completely, ELB can help fire up a new instance in another zone quickly without having to handle re-routing DNS, etc. 2 if you need extra capacity in a hurry (e.g. you just got linked from HN and 10,000 people sign up), you can add more instances without re-achitecting.

Whether it's worth paying for those capabilities is always up to you though.. :)

[captainchaos]

This is a great point. Although it's a slight extra cost, we have adopted this a best practice because it's still cheaper than a more responsive DNS service.