I agree that the addon makers deserve recognition for their work in the readme of the island. Both the addon and the island take considerable amounts of work (especially quality stuff).
As for why make an island that requires addons? I feel that using the latest and greatest only add to the overall quality of an island. It is true that an island maker could simply use original (default) textures and buildings, but I feel that even if the island is of high quality, people will not pay as much attention nor play it as much as if it had the newer models on the island. I suppose an island maker could make his or her own buildings, etc. but that would require even more amounts of time and effort (as well as learing how to make them) - so I guess you have to balance it all and just do what you can.
Perhaps if BIS had released the original games with additional types of buildings, or allowed for custom texturization, it would be easier for island makers to create better islands without the use of other people's addons, but things as they are, I use and plan on continuing to use addons to enhance my island (much like panzerfaust has done.