Personally, when the book is secured, and not available in Mobipocket format, I buy the Microsoft Reader one, and convert it.
I use the LIT version when in multiformat, too, because I discovered that the mobi/prc one wasn't that great, and I prefer the little work converting LIT to PRC (with ConvertLit, then Mobipocket Creator), than suffer when reading...
I stick to Mobipocket for the dictionaries.
Is it really dying ? If you see Microsoft Reader, this format/software didn't evolved since what ?... 2002 ? Nonetheless, it still lives...