In my opinion it seems well worth getting at least a 15 into dexterity by level 11 so a Swordmage can take the Arcane Reach feat, which lets you move the origin square of any close attack powers anywhere within 2 squares of you. This makes sword burst a pretty potent at-will power!
At that point it's not too hard to get the 17 requirement for heavy blade mastery, since you just need to put one point into dex at either 14 or 18 (or 24 if you want to wait), but it's hard to justify the 21 str requirement when you can take intelligent blademaster. I have a firesoul genasi that took the wildfire genasi paragon path, giving me a few strength based attacks, and even with that I don't think I can justify going up to a 21 str.
At that point it's not too hard to get the 17 requirement for heavy blade mastery, since you just need to put one point into dex at either 14 or 18 (or 24 if you want to wait), but it's hard to justify the 21 str requirement when you can take intelligent blademaster. I have a firesoul genasi that took the wildfire genasi paragon path, giving me a few strength based attacks, and even with that I don't think I can justify going up to a 21 str.