Better performance and larger number of monitors supported. Some cons are its usually more expensive than a single card solution and consumes more power. You won't need more than one Titan to max out any game (Besides Crysis 3 maybe).
If you want to use two cards, go with 2x GTX 670's. Cheaper than a titan card.
Don't go for titan! it is a 60 percent increase over a 7970 ghz,(or a 680) with more than twice the price. Go for two 7970s, or two 670's, or if you cant use two cards, a 690 will easily beat the titan.
Typically, two mid range cards are cheaper on a cost per frame basis than a single top of the line card. Sometimes, they are even cheaper outright.
For example, two 560 Ti's costing $410 ($205 each) gave ya 40% more fps than a $500 GTX580 . That kind of performance per dollar made the 560 Ti the best selling DX11 card ever .... even with the new cards shrinking it's share, it still currently holds 4.99% of the market.