Purely for nationalism reasons and because I like to challenge myself to see how well I can play with them. They're actually a pretty complete nation now with a decent collection of bombers, a half-assed ASF, a 'heavy tank' a decent medium and a collection of half okay AA weapons. People on the forums hate the wolverine, but for 35 points and dat offroad speed it's actually my favorite AA vehicle in the game. If one of yours dies to a ground unit, you've made a serious mistake.
Basically, I play it for Stealth bombers+patroit+Abrams. I wouldn't call USA 'easy mode', because making mistakes with an American deck can cost you a lot of expensive units. America really just relies on spotters and then bombing stuff with precision guided weapons, the most expensive tanks and helicopters money can buy you. Just don't make mistakes or you'll be hurting. I like that. Also, the Super Cobra with AA missiles are actually a great mobile infared system and can be a real help when attacking (essentially a superior Chap, with a lot less ammo)
Commonwealth Armored or Mechanized
Commonwealth Armored is actually a lot of fun, even though a lot of their tanks are kind of priced wierd (they're just in an awkward spot) but they have a really good meat grinder deck with lots of decently armored tanks per card. Mechanized is similar, but with infantry. Lots of meat grinderan' you can just throw your weight around.
Blue Dragons Unspec or Armored
I know the forums hate the dragons, but bluedragons are great fun, ASF+SEAD plane is amazing, their tanks are good and really benefit from Elite in armored deck (grenade launcher tank, or the 'cheap' M60 with good pen) They've also got a decent collection of manpads (slightly superior stinger C) and good aircraft (laser guided bomber is cheap and simple, cheap and simple ATGM jet, decent F-15 variant, lots of cheap napalm platforms...) They've got everything a coalition needs to succeed, and I think they've got enough unique units that they're fine. (1400m recoilless rifle teams with shock training are great too; and they come in 150 kmph grenade launcher carriers...)
Honorable mention goes to Commonwealth Support
Even though I hate support decks (they're mostly horrible) I actually ran a pretty interesting and unique support deck that was actually pretty fun- the Commonwealth get the Chimera (20 Frontal armor and 19 pen!) in their support decks which is pretty unique and it allows them to actually not be totally useless in the armor department. As a deck it was mostly about highly vetted AA units and artillery and jets with infantry screens- but it's actually pretty fleshed out and kind of fun. You can do a lot of work with high vetted ADATS and Stormers supported by Marksmen. Keep the skies clean for your teammates- and then you're able to make attacks on your own.