Where isindex is defined (http://www.w3.org/TR/html5/association-of-controls-and-forms.html#attr-fe-name-isindex) it states:
> This value, if used as the name of a Text control that is the only control in a
> form that is submitted using the application/x-www-form-urlencoded mechanism,
> causes the submission to only include the value of this control, with no name.
So an isindex can be the only control in the form.

The urlencoded encoding algorithm (http://www.w3.org/TR/html5/association-of-controls-and-forms.html#application-x-www-form-urlencoded-encoding-algorithm) states on step 4.5:
> If the entry's name is "isindex", its type is "text", and this is the first
> entry in the form data set, then append the value to result and skip the rest of
> the substeps for this entry, moving on to the next entry, if any, or the next
> step in the overall algorithm otherwise.
So an isindex doesn't have to be the only control - merely the first (in the tree).

To me, the former makes more sense than the latter. It also reduces the risk of someone accidentally (eg) using a hidden form field named "isindex" in their form, then discovering that it doesn't show up in the submission as the expected isindex=value pair.