Pos gunning is a powerful defense, providing you have the pilots trained and online.
The problem is that one pilot can only control up to 5 of these very powerful guns, and that currently with roughly 52 days training. Having been the defender in a siege; 1 POS gunner is not sufficient. The prerequisite is currently Anchoring 5, a roughly 17 day train. Horrible.
With the POS changes, this too shall pass. Just in from the forums, CCP have announced that the pre-requsite will be changed to Anchoring 4; making an incredibly useful 3 POS gun pilot in just 4 days.
This means that Anchoring 5 will be only required for Mobile Large Warp Disruptor II's (worth it in null/wh space) and Outpost Construction (null only). If you have Anchoring 5 and are purely high/lowsec, then you will have some wasted skill points. Those that like to bash online POS will have their lives made a little harder. Defenders of POS will be rejoicing.
If the only reason for you planning on Anchoring 5 between now and the new POS release is for POS defense, then you may wish to train to Anchoring 4 and wait.
For the Foo corps, starting with the expansion this gets implemented, Anchoring 4, Starbase Defense Management 3 will become a pre-requisite skill for joining our wormhole corporations, adding an extra 4 days to our minimum skill set queue.
I will update the requirements when the appropriate time occurs.