* Opportunity Attacks
* Bonus damage on many powers (Greenflame Blade, Flame Cyclone, Burning Blade)
* Feat qualifications (Armor Proficiency: Hide, Heavy Blade Opportunity, Heavy Blade Mastery)
Heavy Blade Opportunity is particularly important. Not only can you use an Int-based attack instead of a Str-based melee basic attack on an OA, but you can hit an entire area around the foe (via Greenflame Blade). That means a minimum 14 Strength at 1st level if you want access to it by Paragon tier without using level bumps...12 won't be enough.
Hide maybe, but no hvy armors (not mentioned but... so str can be drastically lower), they are pumping their Int through the roof, and the heavy armors will actually hinder the character. And Hide requires only a 13 str and 13 con.
Weap prof's/feats require a 15 str (hvy blade opp), and 21 str (hvy blade master) so if you are gunning for the feats at earliest possible moment (21st level for the epic tier and 11th for the paragon one) you need only start with a 15 str, otherwise a 14 is perfectly fine.
OA's are nice, but not the meat of the class, the main "OA" style attacks look to be immediate interrupt at-wills, so yet again, focus on Int, and have str at an "ok" number. (with a 14 str, will have a +5 to hit AC or more on OA's)
Aside from extra protection for your ally with Aegis of Shielding, Constitution isn't that important. The only at will attack that uses it is Burning Blade, and the only other power that uses it is Chilling Blow (which is okay, but not as good as Flame Cyclone). You can raise Constititution to increase the protection you can give an ally, but I don't think it's worth losing access to all the other things a high Strength can give you.
Dexterity is also important...I'd try to get a 15 by the Paragon tier, and a 17 by Epic tier, so you can qualify for several useful feats.
Dex is ok also, I would keep at a decent amount, and try to bump it every once in awhile...
With tiefling, I would go with an 18 int (16+race), a 15 Cha (13+race), 14 Str, 13 Con, 12 Dex, 8 Wis (ick, but necessary I think)
This will give the char a decent starting point to work towards getting those feats, a nice bonus at 1st level to hit with his "Int vs" attacks, The tiefling feat for +1 Att/+1 Dam on fire based, with a longsword, should have +8 vs AC on Greenflame blade (with 1d8 + 5 Damage, and 3 damage to all adj enemies)
Not to mention the other powers...