I have been working on a number of SRS’ lately. These do help in the developement of the application. At least you have a formal definition of what the project is trying to do.
A SRS helps in the development of the test plan.
Recently we have found one slight flaw. If the customer does not read the SRS it’s value decreases significantly. If the customer is not interested in reading a large formal document then a few small technical memos would have a greater chance to be read.