mo, where do you find this stuff? and as far as I know a thin white t-shirt carries spf15 protection on its own, so maybe something thicker is already spf30 by default.
According the the SunGuard website and other things I've read (here's one for example: http://www.rei.com/expertadvice/articles/sun+protection.html), a regular white cotton t-shirt has a UPF (ultraviolet protection factor, the rating for fabric as opposed to SPF for topically applied sunblocks) of 5.
I've been using SunGuard on my children's clothes for about 4 years, since they hate the feel of sunscreen and with it in their clothes, it lessens the area of skin I have to cover with the lotion. In that way, this product is a convenience and works for me.
There are clothes on the market that you can buy that have higher UPF; this product allows you to add UPF to clothes you already own. Obviously, you would want to use sunscreen on the skin not covered by clothes.