Hi,
Speaking as someone using a poseidon in a campaign now, I think that it is more usefull underthose circumstances than under any other, although the second time around I would buy it as a reinforcement. The bonus to campaign initiative is usefull, and in the big games that a campaign can produce there is a battlefield role - in a 7p war game, while it remains a big target, it is not quite the priority it would be in smaller games. As reinforcements, it also brings in fighters at a discount.
Having played with it a couple of times now, I'd say the crit vulnerability coupled with low speed is waht nails it. It is very easy to rendert he ship dead in space, with no opportunities for movement. In circumstances where it is the priority target, this means it is pretty mucha write off form turn one. In games where other ships pose a more immediate threat, it can lurk in the background with relative sucess.
FMB