What is the point of Future Sight?
Well Future Sight does damage your opponent, just two turns later. In fact, unlike many status-inflicting attacks, Future Sight isn't quite "wasting" an attack either; two turns later Future Sight will inflict damage in addition to whatever move you make, so it's still possible to attack 3 times inside 3 turns with Future Sight, provided the battle lasts until Future Sight activates.
One of the benefits of Future Sight (from Bulbapedia) is:
Future Sight can hit through Wonder Guard, Protect, and Detect. It can knock out a target that used Endure, but not one wearing a Focus Sash that has full HP. Protect and Detect does not stop from using this move.
A potential setup against a defensive pokemon is to use Future Sight when you expect them to use Protect/etc to save up your attack for later or to hit through a future Protect status attack.
But it's still not that great, since it requires your opponent to be defending either when the move hits or when you cast it. When it's a move with an unusual use that Smogon can't find a use for it's often not all that great of a move (if there's some amazing, broken use for a move, Smogon will find it. Then they will ban it).
An additional use was that prior to Gen 5 it did not take type into account, always dealing a 1x type based multiplier. As of Gen 5 it's a proper Psychic move though, so it's useless as a type-ignoring move. It does however have 100 power in Gen 5 (it used to be 80), making it one of the stronger Psychic attacks (albeit with an annoying handicap). Psychic is only 10% weaker but has zero set up required.
There are a number of good reasons to use Future Sight, especially in Gen II-IV.
The most important aspect of Future Sight is that it will hit a replacement monster. This means that you can cast it against a monster you can defeat easily in the first round of combat, and use the damage on a later, more dangerous monster.
In Gen II-IV, the damage dealt was typeless, and based on targeted pokemon's Special Defense, NOT the hit monster's.
And, as Ben says, it can strike through a number of defensive moves.
This is a move that's only really useful in trainer battles, but it has a little niche that might be hard to otherwise fill.
Source: http://bulbapedia.bulbagarden.net/wiki/Future_Sight_%28move%29