Personally I much prefer to keep vars from any source together in their own place. The HTTP*VARS are perfect for that.
If you use register globals you can access the vars outside the HTTP*VARS arrays, but that also means you loose the link between the var and it's origin.
Was this a POST/GET var? cookie var? session var?
So you move to naming conventions and you end up with variable names like $postvar_thisvar, which is well... irritating, especially if you want to use a postvar in a url, becuase then the postvar becomes a getvar, and changes names...
With the HTTPVARS you can also simply pass the entire array to functions, saving you lots of typing.
If you have a form and you want to send all the data from one page to another,
the HTTPVARS are great because you can simply serialize it, or loop through it and create hidden fields for all it's entries.