Both 'for all it's worth' and 'for whatever it's worth mean ; if it has any value. Usually it implies lack of confidence. Therefore, "Allen was a poet, for all it’s worth" implies that Allen was not a well regarded poet and that the poetry was not considered to be any good.

'For all it's worth' has different meanings. It can mean a) to get the maximum out of something b)with great energy, effort and determination. or c) if it has any value/ but it may be worth nothing (usually implying futility or a lack of importance). I think in your sentence the meaning is the more negative one. 'Allen was a poet, for all it's worth. (even though that may not have any value/ meaning/ importance!)

Worth=importance