At later levels Summoners become pretty powerful... but to start they are weak, and as Omi said, healers. They have spells that can buff the whole party, like Blinkga, and Enthundera, and Stoneskina
As for PVP... I have no interest in it what so ever. Ballista is fine, no more is needed. If they do implement it, they should make it to where both players will have to accept, but even that would be a waste. Whoever has the best 2hr wins, wow. And nobody is going to engage with somone who is stronger. (Also, some american players have reached 75, and many MANY Japanese players are at 75.)
Hmm... as for the spell Holy? I've never seen it do more damage than half the MP that is used to cast it. Nuff said
As for Job/Subjob job traits... some don't stack. If it's the same ability that two jobs share you won't get the bonus of both. The Job trait "Clear Mind" for example, you only get the first version of it once. This may be confusing to you now, but you will understand if you become a mage.
There is a new system out where a 3rd job can affect your attributes, but you have to be level 75 to use it.