I'm not talking about lvl 6-7+ in this case. You already pointed it out that your base AD will greatly increase your last hitting capability.
I was referring to Spellsword being a last hitting assistant at lower levels, just like other masteries, which are more AD focussed rather than AP, making them less desirable to pick if you look at them in the big picture (early > late game). It could allow you to ignore the Butcher, and Brute Force masteries.
Assuming you've got a decent AP rune build, you'll start with 10-25 AP from your runes, plus 7 raw AP from your masteries, and not to forget the 5% AP increase.
You'll gain 1-2 damage at the start. Which is about the same as the spending 1 mastery point Butcher or Brute Force. This only gets higher with the amount of AP you've got. I'd say it's a fair pick compared the other options you have.
Except, hybrid Teemo also has a fair amount of AP in his build?Except I said it's useless if you're going to go hybrid. If you have 500 AP you are not a hybrid.
---------- Post added 2012-12-04 at 07:44 PM ----------
Well that's a bummer