I hear what ya saying, thats why i built 2. One with PVC and the other with cardboard. I can't really see/hear any difference in them at all. I have SD composite inside both tubes and they have been sealed and covered with carpet. PVC that is 3mm thick or fibreglass that is 3mm thick is still 3mm thick at the end of the day. I know that they both have different material bases and what not, but if you make a tube out of cardboard or out of PVC then the same principles apply with the enclosure. You still have to have the same volume for the woofer, it has to be sealed and tight and some sort of SD inside to stop the resinance. They both have there pro's and cons but when it comes to the end of day you go with with whats going to work the best and cost the less. Fibreglass is very expensive and PVC/cardboard isn't. Its very easy to fix if you make a mistake with PVC/cardboard and it is easy to a degree with FG but you have to wait for it to dry and then that requires sanding, and starting again.
I think it comes to personal preference, but after talking to some pro installer/builders and the bulid instructions and materials that they gave me i can't complain and i think it will work well.
What ever floats ya boat at the end of the day