Well, this is a good question and most of us will likely answer 'yes of course'. Indeed, as a SysAdmin you want regular tasks to be automated and act the same way on a bunch of systems. But what if you need a custom task (that you only need once) to be launched on some machines at the same time ? A lot of solutions exist of course to "push" commands to be executed by clients machines. Some will use RHN/Satellite, some will prefer using something likeCFengineor Puppet. I've even discussed with some admin pushing some 'encrypted' commands on a Twitter feed followed by some clients able to understand the commands and process them ... Multiple solutions exists, and your imagination is probably the limit.
But what if you just have to manage a *very* small amount of servers at the same time for different customers/environments. For example, as an IT consultant, you probably have a bunch of customers running different solutions and sometimes with only 10 or 20 servers, right ? Will you install a satellite proxy server just to push commands for those 15 machines ? or sometimes even less ? IMHO, ssh is the solution, especially if you want interactive output/processing on all the machines. I discussed with a friend of mine who said that ssh was the solution but taking a shell on 15 servers "one at a time" was time consuming. Of course it is. So why not using a shell multiplexer or distributed shells ? I was astonished to see how many people I had the chance to speak with don't even know that you can launch interactively or in batch the same commands on multiple systems at the same time ! So I thought that maybe it was time to write (like many others did) about ssh based solutions !
I've tested and continue to use on a daily basis different programs (that use ssh in the background of course). Depending on the situation (you'd have to test them to find the one that fits your needs), I like to use cluster-ssh, mussh and shmux. Others are available, like pdsh, etc .. but the first ones are the ones i'm more comfortable with and that I personnally use.