See what the Swedish organisation for Dyslexia wishes for a good and usable website (in swedish).
The search feature is important from an accessability perspective. Visitors with special needs use the search feature more often than other users. Hence, it is very important that the result from a search results in a match, even if the user did not spell 100% correct. A decent search feature must include a working spellchecking service.
Users with reading- and writing difficulties and dyslexia usually have more needs of a good and functional search feature to find what they are looking for. Worth noting about this is that about 20-25% of the population have some kind of reading- and writing difficulties.
Even immigrants and users that aren't familiar with the language yet, rely on the search function to find what they are looking for. If you have difficulties with your reading, or even the language in general, it will be very hard to "scan" alot of text to find what you are looking for. For these users, a spellchecking service included in the search feature is a must.
A good search feature includes a spellchecking service that helps the user to find what he/she is looking for even if he/she doesn't spell so well. A perfect search feature also combines the traditional spellchecking service with phonetic matches to assist the user.