I am not sure how useful Aspect of the Viper was before the Wrath of the Lich King came out, but I sure find it useful now. It used to be that in the middle of a prolonged fight or series of fights, I would run out of Mana, and would have to rely on potions to restore it. Of course, that meant that a healing potion would be unavailable to me for a while, and that is not a safe thing to have happen. So I would end up leaving my Mana at low or zero, and then I would be unable to trap, or Mend Pet, or do any other useful stuff.
By switching to Aspect of the Viper I actually let my attacks generate Mana for me, as the Serpent Sting and Arcane Shot, as well as a few others, will actually increase Mana instead of use it up. In fact, if I keep this aspect on then my Mana simply stays at or near full and I can use spell after spell without fear of running out. The downside is that total damage is reduced by 50%. But if the pet is doing a lot of the damage in the first place, and if you are not running out of Mana, then this is a decent trade off.
I like to use this in long grinding situation, especially if I am taking on no more than one or two mobs; in other words, when it really does not matter how long the kills take. I end up more than making up the difference in the fact that I don't have to pause to drink or to mend the pet after Mana regenerates.
In bigger fights I might switch back to Aspect of the Hawk in order to take down lots of mobs fast, especially a boss, but afterwards switching again to Viper can help regenerate badly needed Mana in order to finish the fight. The last thing you need to have happen is to not be able to cast a Mend Pet when your pet is tanking the last three mobs for you.
I have also found that Aspect of the Viper reduces my aggro in relation to my pet's aggro. I am not sure if this is true or not, but I have found that my pet will hold aggro a lot more than when I am using the Aspect of the Hawk. Since my total damage drops, and my pet's does not, this make sense; but again I have not been able to get verification on this.
Lastly, if you have a bit if a run between spots, switching to Viper can help you generate your Mana, as you will get 4% of your total Mana regenerated every three seconds. That's 75 seconds for a full regeneration, so you can spend the time running instead of drinking. I do this only in situations where Aspect of the Cheetah or Pack is not safe.