You would think that with the huge evolution that databases went through in the past years multi user testing is out dated. But that is completely untrue.
Web applications now days face a user pool of more then thousand or even more then tens of thousands of users. These are a lot of people acting all at the same. There ought to be some problems. Imagine a bunch of users requesting information on one subject. Or editing a document? Take a document from a shared library and begin to edit it and then submit the change? Who wins? First come first served? Or imagine a document with the same name submitted by a dozen of users at the same time.
Now days there are keys that prevent these kind of errors from happening. Unique keys that don't allow conflicts. Even so a deadlock is possible. Imagine a tunnel that only allows one car at a time to pass it. Two cars wont fit and when one car comes from the other and then it's a lock. Now this is possible if the design was not good enough or the developer did not thought of it. In my career i encountered multi user problems all the time when testing Web applications. They were never solved only patched.
So don't underestimate multi user testing because when a deadlock occurs that's no fun...