Secondwind offers one additional heal for the group, but it is only able to target yourself. Baring some attack and utility powers, leaders get two free heals to distribute at some point during the combat so if nothing else by using it you are essentially getting HP back that does not use some other limited resource (eg: leader heals, potions) at the cost of a standard action on yourself. This frees up those other resources for use elsewhere and if you do have a leader, they will likely thank you for it.
Also frequently, there will be turns where there are no very good combat actions to take, and more than a healing surge of damage has been done to you. Using a second wind in these situations is a great idea as you'll have more health for the fight, and if it turns out it wasn't critical, that is one less healing surge you must spend in the short rest afterward. No long term loss.
Even if the heal you use on yourself is immediately countered by an enemy attack, that's one more attack that you will survive, giving everyone in your party additional time to act.
Note: This is all of course ignoring the additional benefit of the +2 to defenses, since the question was mostly about the healing aspect and the +2 is an added benefit to what I listed above.