Older version's still serve a purpose for someone somewhere, so they wont be removed. That idea also was discussed in other threads.
MSN, AOL, and most chat providers do force upgrading the clients, to better work with the newer server software. This is for many reasons, and most importantly its for security, such as holes discovered in the cleint or server software.
Sometimes they decide to switch from using C programs to XML. There are many reasons why they force updgrades which as long as the service remains free to use, i do not see a problem with upgrading. Now, if you were paying for the service, and each upgrade forced upon you ended up costing you more money, then i could see people being frustrated.