Concurrent Dictionary Correct Usage
It depends on what you mean by thread-safe.
From MSDN - How to: Add and Remove Items from a ConcurrentDictionary:
ConcurrentDictionary<TKey, TValue>is designed for multithreaded scenarios. You do not have to use locks in your code to add or remove items from the collection. However, it is always possible for one thread to retrieve a value, and another thread to immediately update the collection by giving the same key a new value.
So, it is possible to get an inconsistent view of the value of an item in the dictionary.
Best way to find this out is check MSDN documentation.
For ConcurrentDictionary the page is http://msdn.microsoft.com/en-us/library/dd287191.aspx
Under thread safety section, it is stated "All public and protected members of ConcurrentDictionary(Of TKey, TValue) are thread-safe and may be used concurrently from multiple threads."
So from concurrency point of view you are okay.
Yes, you are right.
That and the possibility to enumerate the dictionary on one thread while changing it on another thread are the only means of existence for that class.