I think I had this discussion elsewhere. The reason summoners don't get great armor and wikkid DeePS is because of the nature of their combat. Instead of engaging the enemy, you're supposed to avoid them entirely, while your summons do all the work of combat. Counterintuitive, yes, and not very liberating for the Dovahkin crowd, but then if it's not for you it's not for you.
With the summoner completely (as much as possible) out of combat, their damage load is greatly lightened, and they no longer have to race the enemy to 0hp. Thus they can sit back in safety and let the minions whittle the enemy down to a toothpick. Hence no need for high DPS or armor. Anyone that's finding they can't manage to keep from charging straight into the enemy, or can't get their summons to hit them, simply needs to adjust to the summoner playstyle.
I also saw it suggested that you should try to back up your summons' damage with ichor (not many enemies can shrug off losing 20 defense), as well as indirect damage such as nimbus rod, land mines, starfury, bee gun, basically anything you can throw around a corner and down the hall, which is where a summoner should be for all fights.
A lot of this is a little contradictory to the way many people play Terraria but I personally wish each of the other combat styles were as different from the others as Summoner is from them.