Strength helps with your natural healing rate, melee armour, and conditioned body, so boosting it a bit isn't a bad idea even if you don't need the melee damage bonus.
There were times in dragon form that I didn't need to heal very frequently, but not so much I considered removing the +40% HP armour. Even for the couple places where I tried using the shield and summon spells, etc, I didn't have a problem with mana that I would consider switching the HP bonus to mana.
No, the difference between those cuirasses is purely cosmetic, even in the original release of D2:ED. Some people prefer using a complete set, or just like the look of one cuirass over the other with a given bonus.
