SSDs can be very useful in servers. As with all storage technology questions, it depends on what you plan on doing with them.
SSDs are supreme when you need to be sure that the latency between I/O operations is as low as it possibly can. Enterprise SSDs, especially devices in the "cost is not an issue" price range, have been around for coming on a decade now. In markets where cost can be an issue, they're still a new technology and the market still hasn't figured out the answer to the question, "how reliable are they compared to rotational media."
First of all, you need to know what kind of I/O patterns your server will create, and how sensitive they will be to storage latencies. If it is highly random and high volume, SSDs will allow you to supply that load with much fewer drives than rotational media. If storage unit count is also important (say, you only have so many rack units and an addon shelf of 24 15K RPM SAS drives won't fit) SSDs can help you stay within performance envelopes.
Second you need to know how much storage you need. SSDs are great for performance but they still barely hold a candle to rotational media for sheer capacity. Things start to change if you need both performance and space. If you need multiple TB, that 24 drive 15K RPM SAS array starts looking really attractive.
Do SSDs eliminate or reduce the need for RAID? No. Drive failures still happen. For high-write applications they can happen faster than for rotational media, especially if you're hoping to get 5 years out of the hardware.
Are SSDs appropriate for the OS volume? They can be. If your swap volume/file is on that drive it can provide some nice improvements in speed. But you still want to at least mirror that volume.