The Ramban's explanation was accepted, largely because it is difficult to be sure of what the Rambam really meant and how it would work in practice. The difficulties of counting on a d'orayta level, even according to Ramban, were compounded by the lack of expertise in assessing which colors of blood rendered a woman temeiah and which did not. A woman could bleed six days, thinking she had gone through six days of niddah when in fact she only began having blood that would render her niddah on day five. Counting and differentiating between niddah and zavah days would thus be more difficult. There could also be an issue in assessing zavah days with blood that was seen bein hashmashot.

The Rambam's view is most interesting on a theoretical level and in order to bring into relief the implications of the Ramban's view (for example, that one cannot be a niddah and zavah simultaneously). We would not recommend getting into the precise details unless teaching in great depth.