Your short answer is "no", you do not absolutely need the Computation Module.
Perhaps a little clarification will help you see things in prospective.
There is a reason why I and many others don't want to relay on "JS" is some visitors could have their JS turned off in their browser. It's a security thing. JS scripts normally run in the user's browser rather then on the server. This means the script is run on the user's computer and any time you allow an external script to run on your local system, it creates a potential security hole. In other words, some one could write a harmful JS that could harm the user's computer.
With that in mind, if you are serious about wanting your forms to work for everybody, you don't want them to be JS dependent. JS is great to add some flare to any form or web page, but to handle critical tasks, php is a better choice.
Php runs on the server rather then in the user's browser making it not browser dependent like JS.
Back to your original question; Russell had developed formmail and the Computation Module so we don't need a heavy knowledge of php. He has done the legwork for us. If you know php scripting then you can do whatever you want with conditions along with many other functions without Computation Module. Or, you can use JS if you choose.
I like the Computation Module very much for the main reason is it DOES simplify a lot of tasks.
JS has the option to update a page immediately where as php needed to load, unload or refresh a page to show the update.
Hope this helps sort things out