I agree 100%. For example factoring, for reducting fractions. The student who does not know the division facts at the 100% level, ie (21)/(3) = (7), will have a great deal of trouble reducing the fraction (14)/(21). In my work with high school students, I find a large number who do not know the multiplication and division facts (0-12).
People who don't want to teach fractions, don't understand where fractions fix in the big picture. Look at probability. The smallest probability number is 0 and the largest probability number is 1. All other probabilites are between 0 and 1, ie a fraction or a decimal. If the student is not completely proficient with fractions and decimals then it is a waste of time to study probability, the student can't work any problems.
Most of the math taught in grades k-8 has this prerequisite relationship. In the development of AMATH we reverse engineered the prerequisites for Algebra I and developed 97 modules covering k-8 math. The program insists that the student master each module.