How to take advantage of AWS EC2 instance store (SSD) in a database environment?
The ephemeral disk is excellent for exactly the two use cases you mentioned, swap and /tmp.  Don't underestimate the performance benefits, here -- the ephemeral disks are not only SSDs, they're also physically inside the host machine, unlike EBS, which is network-attached, so you get high bandwidth and low latency to the storage device.

Otherwise, their applications are somewhat limited for "normal" database setups... but in certain other applications, anything where the machine itself is also more ephemeral by nature (QA/dev environments, scalable read replicas), they can offer excellent performance.

If you're the experimenting/creative type, you might toy with a RAID-1 array split between EBS and ephemeral (which could theoretically improve your read throughput without jeopardizing durability).


  seems to be a waste of the money spent on reserving the SSD storage.


Ephemeral disks don't cost extra -- they are included in the price of the instance, so the cost for a given instance type is the same whether you activate them or not.

Incidentally -- always enable them when you launch an instance that includes them, because you can't activate them later.