I think that the primary purpose for the ports is to allow for ventillation. It would seem to be a bad idea to have those areas sealed off forever with entrapped moisture and pressure differentials. They can also be used to store small items.

   Definitely useful for storage.  You'll be glad you have them, afterwards, to store, e.g. sail ties, tow rope, plastic wedges for the mast well, spare sunglasses, sunblock, extra misc lines and bungies, spare notepad and pen, binoculars, etc.

   I chose to install the ports when building my skerry. I would not put them on. Hear me out....

Last spring I installed two big hatches, these are still watertight but far more easy to access, these are on the seat benches that I already have padding on.

The inspection ports where great and allowed me to store a little bit. But when a jerk ran into my rudder and tore out a small hole where the rudder used to be I could not open the port while underway to bail out the aft section. It also took me a bit to figure that there was a screw sized hole below the waterline.

If the ports are all you will do they are a must and you should spend the money on them. I reccomend going all the way and having better access.