Another reason for *filed* alternates is lost comms.
If you turn up at your destination in IMC w/lost comms & unable to land, ATC will expect you to proceed to your filed alternate.
Otherwise I agree about the arbitrariness of *filing* an alternate.
Unless crossing national borders or tricky airspace, in a diversion situation & with ATC's help, I'd always divert to the most appropriate alternative, irregardless of whether I'd put it into the original flight plan or not.
(but I'm sure there's situations where this could cause major administrative trouble)