I have had experience dealing with multivariant and a/b testing while using Omniture. You could almost certainly do everything that these testing suites do yourself, but you would have to build an awful lot of infrastructure to accommodate the variants and collate the relevant data and display it back in a meaningful way to those who are interested. Given the sheer volume of options that any of these types of testing suites come with, it would probably take a great deal of time to replicate this type of function on your own.
If you have the budget, I would recommend using one of the many tools that are specialized in doing this sort of thing. Of course, if you don't have the budget but have the time, I can see where building it yourself could be a lot of fun...
I'm not sure how much of an 'answer' this is more than just my opinion.