As DM, I find swordmages to be very annoying, second only to fighters, I can normally ignore or get around wardens and paladins quite well. But in my experience this also depends largely on the player. I've played alongside a completely ignorable swordmage, and a warden who was impressive at limiting the enemy's options.
There are a lot of factors to consider. Swordmages shine in encounters with a few powerful enemies and lots of minions or few lesser grunts, but in a skirmisher/artillery heavy encounter with lots of weaker enemies with no clear powerful opponent, they are not so good, while a fighter or warden might be able to exert exceptional control in those situations. Terrain can be problematic to a paladin or fighter, while swordmages and wardens may have an easier time bypassing it. And teammates who don't stay put and keep shifting the battlefield are annoying for fighters and wardens, while paladins and swordmages can do their jobs easier.
It's obviously not all clear cut who would do best in what situation, a lot depends on the build, power choices, and such, but I think the swordmage's defender role is acceptable at worst. I probably wouldn't play one as the sole defender of a party of 5 or 6, but for a party of 4 it's quite sufficient, and as a secondary defender in a party of 6, it can add great flexibility.
[MENTION=65726]Mengu[/MENTION] is right. The power of a [shielding] Swordmage is highly dependent on player experience. Unlike a fighter, whose tactics are mostly about the most effective way to get into an enemy's face, a Swordmage must be prepared to adopt radically different tactics in order to be effective in various situations.
Against solo enemies, a Swordmage can be devastating. Swoop in, Mark, *Run Away* [double move with Amathor's step and minor action mark in the middle]. Now the monster can bounce between you and an allied melee striker, taking all kinds of OAs unless it stays put and does nothing, which it will not do. You can also create situations where not only is the Solo unable to damage your allies, but is unable to even attack you. just brutal, especially if its attacks don't have decent riders.
Against minions and skirmishers and swarms, radically different strategy. Now you want to be in the thick of combat, laying down multiple marks and Swordbursting at almost every opportunity. You want Intelligent Blademaster and Bracers of Mighty Striking to punish enemies who attempt to disengage as much as your meager hitting power allows.
Against a mixture of enemies, things get more difficult. I think Swordmages get a bad rap because most encounters are of this type. This situation is really a mixture of the situations described above. You begin with the bottom strategy and then as the encounter goes on and the number of enemies starts to thin out, you transition to the top strategy. The trick is knowing when to initiate the transition.
Swordmages are best against solos and small groups of enemies, but as described above they do have capabilities against larger clusters of enemies with their burst attacks and multi-marking powers and feats. The problem Swordmages have in these encounters, and that [MENTION=93444]shidaku[/MENTION] describes, is that they are not sticky and their multi-mark punishment can be overwhelmed due to the immediate action limit.
Dealing with these limitations takes some skill and careful power selection. I think Frigid Blade is a useful at-will for dealing with these situations. If you're Shielding, you're going to have a high Con and with a high Con this power can virtually immobilize a threat. Hit an enemy with it and shift, teleport or 'risk' an OA [don't worry, you have a high AC] to get in another monster's face. Lather, rinse, repeat.
Even with these stupid Swordmage tricks, something *will* eventually get through, and since preventing damage doesn't deal with riders, you're going to want to use Transposing Lunge [the shielding version] to prevent initial attacks altogether. After you've burnt that, Dimensional Vortex can help prevent or delay protracted melee situations that damage reduction can't handle.
This is all stuff I do very regularly when playing a Swordmage, and it works. Indeed, the number 1 problem i have playing a Swordmage is taking loads and loads of damage from being constantly assaulted by the multiple monsters I have marked and slowed. Swordmages have relatively low HP for defenders and no healing based utility powers so things can get dangerous when the monsters decide the best way to get at the squishies is to plow through the squishiest defender.
All I can say about that is to have good defenses and to not be afraid of wearing Parry Gauntlets and repeatedly going into Total Defense if you start getting mauled.
Which brings me to the OP .. 32 AC for an effective Swordmage isn't a lot. Compared to the 30ish average AC for an out of the box Fighter, that's only a 10% less likelihood of being hit at level 12, and multi-marking Swordmages can draw much more than 10% more hits than a fighter if they are doing their job well. they'll also have at least 10% less HP. Recipe for disaster without high defenses.
Every turn playing a Swordmage, it is possible to make a disastrous mistake. Either in terms of getting your relatively low HP get beat down because you took a gratuitous swing at something instead of Total Defense .. or because you went into Total Defense instead of increasing your stickiness, allowing your buddies to get swarmed.
But if you think very carefully and make the correct situational decisions and don't overestimate your abilities, your Swordmage can be an incredibly effective lock-down machine.
