Oftentimes one can see T2 pd used offensively, given that it is superior to equal cost moving units.
Examples of that is the turtlecreep maneuver or when an ACU is the first and only unit approaching a choke point
to secure it with PD, not by attacking with some units. Extreme example is the T3 ACU hopping out of a transport all alone in the enemy base. Also when there is a firebase relatively close to an opponents
main base or some other cruicial point on the map, there is little choice then to block the firebase expansion with shields
and PD, which results in a no man's land for units up until the T4 stage and potentially a PD standoff that consumes a significant portion of available resources but gives no benefit except not loosing the standoff.
If the build time of point defenses was bigger, it would give more time to counter the forming of a firebase with lower
tech units, would give more time to plan a counter strategy that is not just an opposing firebase and it would also require
to first take control of a piece of land with mobile units before a base can be established. This would still make point defenses worthwhile and superior against an equal cost mobile army, only slower to establish on the battle field.