Seeker Missile used to be much more popular in beta. Since then they have lowered the damage and required research to unlock it. The combination of these with the fact that seeker missile is slow, dodge-able, and takes a ton of energy to use has made it much rarer.
There are often more effective use of a raven's energy that take precedence over seeker missile, though a well placed seeker missile can still do a lot of damage.
A seeker missile does
100 damage to a radius of 2. This can be very devastating against clumped units.
In terms of capabilities to do damage, consider this: for 50 energy (instead of 125) you can throw down auto turrets. A non upgraded turret lasts 180 seconds and shoots every .8 for 8 damage.
(180 * 10 dps) = 1800 damage
You can throw down 2.5 turrets / seeker missile =
2.5 * 1800 = 4500 damage.
This is the equivalent of hitting 45 units with a seeker missile! That is un-upgraded
Upgraded turrets last for 240 seconds and benefit from weapon upgrades leading to:
(240 * 13.75dps) = 3300 damage
You can throw down 2.5 turrets per seeker missile =
2.5 * 3300 = 8250 damage!
This is the equivalent of hitting 82 1/2 units with a seeker missile!
You can see why turrets are a better raven damage dealing solution.