I tend to agree with that....If 18-year-olds are going to be accorded the rights of adults (as per the 26th amendment), then they should be accorded all rights, not just selective ones.
The other end of the argument is that 18-year-olds are not emotionally mature enough to legally drink at that age. I would counter that it depends on the individual, for I know people in their 30's who aren't emotionally mature either.