<fieldset> still gives you the ability to place <legend> elements, a great usability feature. ;-)
It's doubtful you're going to need multiple fieldsets for SearchForm, but it's also nice to split up different sections of your form if it's for anything else. For example: 'Contact' fields would have it's own <fieldset> element, and a <legend> element too, so it labels that 'set' or 'group' of fields logically.
It really should be used, plus it also does give you more flexibility in styling forms with the extra element, like different background styles... ;-)
All forms created using SilverStripe include the <fieldset> element as Ingo said, and as such it was probably just 'forgotten' in the SearchForm. Either we leave it for 2.1 or we fix it in the 2.0.2 or 2.0.3 subsequent releases, and just add a note in the changeset that we've done this for validation reasons, and W3C compliance which is important. :-)