The idea of the Laser Trip Post or (LTP) is that you can set up a dual post system that will detect vehicles between each post and fire a powerful laser to do moderate damage.

Stats: Laser Trip Post: Eden and Maesis
Post group Totals:
HP: 200
Armor: Light
Ability: Hidden in fake brush netting and is mostly burried in the ground
FP: 1 Advanced Laser: FP: Medium/ AP: Medium/ Range: Short/ ROF:Medium
Size: 1 square y by 3 to 6 squares x: 2 center squares are passible and are the posts firing area
Common Ore: 600
Rare Ore: 20
3rd Ore: N/A
Power: 5
Colonists: 0
Tube: NO

This makes NO sence at all as GPs will automaticly shoot any enemy they dont need to drive through a lasers beam for it to go off.  And such Defence is very very easy to get around or disable.

It designed for use in narrow valley entrances! And also they won't see what is shooting them so how can they avoid it?

by blowing it up before they cross the line.  If they dont cross the line it wont work.  And you would basicly use a EMP and a Thor/ESG GP at the end of any choke point.  That would stop people almost indefinatly.