My argument against it being purely illness is....

if it's illness, then why it's not the same for everybody? I mean look at two illness the illness model advocates love to use: diabetes and cancer.... these have very similar symptom with everybody. And easily measurable. It had same symptoms throughout the ages too.

With bipolar... it's too different for everybody. And how it is experienced changes with time too. Sure personalities come play a role here. We don't act the same when manic or depressed....those states only enhance and play with our existing personalities.