I love my assault swordy, although I completely agree that shielding provides better defense. Assaults have been described as dissuaders more than defenders, and I'll cop to that. I focus my swordmage on the strong striker seconday role and a crazy teleportation shtick. My assault mark is only part of what my character does; what I'd like the most for better marks is more of them at once, which I'll eventually get.
What I do have is often the best AC at the table, plenty of hit points and surges (CON secondary, self-heal encounter power), decent attacks, and fantastic mobility. Difficult terrain? Cool; slows them down and not me. Slowed? Somebody remind me to roll my save next round. Grabbed? ORLY? Are you sure?
If I need to open against artillery way in the back: fey step 5 squares to front line; mark some critter; dual lightning strike another 5 squares; and if needed, action point + armathor's step for 5 more squares; "hello, Mr. Archer!" With eladrin swordmage advance, thats 3 weapon attacks (+14/+11 @8th level) and 15 squares of teleportation. If its really important to bounce back an forth repeatedly from the line to the backfield and back, careful use of marking and dimensional bond handles that.
This character can be a bit situational, but he's also very flexible, so there aren't too many combat encounters where I feel like a spare. When my mark is mostly redundant, I focus on damage and bypassing defenses; burst and reflex implement attacks can make me very popular; some LFR mods use LOTS of cheesy swarms.