The order in which Trooper's gain access to their companions gives them a lot of gameplay style options early on. The first companion is DPS and the second heals, which allows the player to settle into a gameplay style by around level 15. Using a DPS/tanking companion provides a very different gameplay than tank/DPS healer. DPS/tank with a healer might take a little longer to get through quests and mobs, but you definitely die a lot less, whereas using two DPS or even a semi-tank/DPS combo might allow you to kill things faster, but there are a lot of circumstances where you're waiting for the resurrect timer due to miscalculations or accidentally attacking more than one elite.
The Trooper companion line-up has good variety and there is a hot chick as the healer which is great for players wanting a romance storyline with a female character. Not only does companion affection help your relationship with that companion, it also unlocks valuable conversations which grant a lot of experience. I didn't learn the value of gifts to companions until I had already reached high level 40s on beta.
Check out a compilation of everything I have been able to gather about Trooper companions and their gifts here: