Put differently:
Is there a good reason to choose a loosely-typed collection over a type-safe one (HashTable vs. Dictionary)? Are they still there only for compatibility?
As far as I understand, generic collections not only are type-safe, but their performance is better.
Here's a comprehensive article on the topic: An Extensive Examination of Data Structures Using C# 2.0.
With regard to using non-generic collections for storing heterogeneous collections of stuff, you can always use List<object> to accomplish the same thing. For this reason alone, I'd say there's almost no reason at all to touch the non-generic collections ever again.
The exception to this would be to maintain compatibility with systems written in other languages, or against previous versions of the .NET framework, but that's a pretty "edgy" case if you ask me.