When should I add life gain cards to my deck in MTG?
Have you ever seen any of the decks that were popular a few years ago circa Coldsnap, that centred around the Martyr of Sands/Proclamation of Rebirth engine?  If you're going to gain a lot of life, then a combo that can realistically get you 10-20 life every turn is what you need to be looking at.  A lot of decks - any deck that wants to win by beating down with creatures and burn spells - have no defence against such a strategy.  (Of course, not all decks are like this.  You'll need to have a different route to victory against other combo decks.)

Here's a sample budget Martyr list that I found on the official Wizards site, in an article from 2009:


  4 Adarkar Wastes, 7 Island, 10 Plains
  
  4 Martyr of Sands, 3 Ranger of Eos
  
  4 Muddle the Mixture, 2 Proclamation of Rebirth, 4 Thopter Foundry, 4
  Sword of the Meek, 4 Thirst for Knowledge, 4 Condemn, 1 Journey to
  Nowhere, 1 Ghostly Prison, 4 Path to Exile, 4 Fieldmist Borderpost


Lifegain is a good strategy to consider in any environment where fast red decks are dominant - and this is not actually that uncommon, given that fast red decks tend to be a cheaper option in Magic terms.  If rival decks are happy to spend a card and one red mana to aim a Lightning Bolt at your dome, you will be very happy with anything that efficiently gains life against them, as this efficiently undercuts their strategy and makes it very difficult for them to win.  Just be sure that your lifegain kicks in pretty quickly, as left to their own devices many red decks can easily deal you 20 points of damage by turn 4...