> Are there any real world examples which show the benefit of supporting 
> negative indices?

A common case is ignoring parentheses at the beginning/end, e.g.
>>> re.compile('[^,]+').findall('(foo,123,(),bar)')
['(foo', '123', '()', 'bar)']
>>> # ignore the surrounding ()
>>> re.compile('[^,]+').findall('(foo,123,(),bar)', 1, 15)
['foo', '123', '()', 'bar']
>>>
>>> # extract attributes from a tag (poc, doesn't handle all cases)
>>> re.compile('[^ ]+').findall('<input type="checkbox" id="foo" checked>', 7, 39)
['type="checkbox"', 'id="foo"', 'checked']

In both cases using -1 as endpos is simpler.