My reasons for preferring longer-life developers are:
(1) convenience of not having to mix each time
(2) environmental concerns... less waste
My reasons for preferring one-shot mixes are:
(1) highly predictable results
(2) the particular characteristics i.e. from amidol, POTA etc. POTA is probably the shortest-life developer I've met. I wish I didn't have to toss it after 1-2 hrs but c'est la vie.
Generally my feeling is that people probably worry too much about developers "going bad"... especially paper developers. This leads to premature disposal, which is unfortunate. I find that PQ retains good characteristics even after 1 yrs, simply capped and with no special vacuuming or whatever. Likewise D76/ID11, in concentrated form. I was also rather surprised to find that wd2d+ part A and B last at least 6 months, stored separately and without any special treatment... just sitting on the shelf in capped brown bottles.