Another thing to take into account is that, in the archaic age particularly, armies would often meet on the field of battle and then agree to settle the differences by having one warrior from each side meet. This form of battle would have been more akin to the gladiatorial matches of later periods. Since the Greeks, and even the Romans to some extent, had a romanticized concept of naked warriors it is quite possible some of these matches might have taken place naked. By the classical age, these one-on-one contests were not as common. Soldiers would have wanted to wear as little as possible, especially if the weather was hot, but they would also want protection.
Hoplites usually wore greaves, vambraces, and a chest-plate. They would also carry a shield and spear, with some carrying a short sword as a secondary weapon. I have read accounts that the Spartans would occasionally cast aside their clothing and fight naked if they wanted to show total scorn to an enemy that they did not fear. Maybe there is something to that, but I have yet to see anything conclusive from primary sources that indicates this ever happened. On the other hand, while art is usually just art, sometimes it is a window on the society. Some ancient pottery art I have seen depicting the Spartans, while showing them with shields, helmets, and spears, shows them with nothing else on except a garment that covers about as much as the loincloths in 300, though of a different design.
I came across a reference in Plutarch's writings (Life of Lycurgus) which seemed to indicate that Spartan men wore only one garment on a regular basis, and that this garment left them bare above the waist. The only references made to their complete nudity was during their exercises and their games, during the latter of which the young women and girls would strip themselves and join the young men and boys.
Plutarch also mentions that Spartan boys learned how to march barefoot and go naked during their training. Plutarch mentions that at the age of twelve their undergarment would be taken away and only one upper garment given them each year. This garment was a red cloak, which they would wear as Spartan soldiers once their training was complete, along with whatever armor was issued to them. The cloak itself, however, was not worn during battle, as it would be a hindrance to movement.
Plutarch does not mention whether or not the Spartans wore any other clothing, so it is possible the Spartan soldiers may well have been naked in battle other than their armor. I have seen it pointed out that they would not have wanted to wear metal armor on their bare skin, yet we know that they would line the inside of their armor with a layer of leather. Keep in mind, also, that their shields offered a great deal of protection.