It's based on a lot of research that says you only have to learn how to read once. Kids who learn to read in their first language will transfer those skills to English faster than if they try to learn to read and learn English at the same time.
In my opinion, it depends on the age of a student. If a student is really young, for example, 1st grade, it is okay if they cannot read well enough in their native language. I still think that their unability to read in their native language will not effect their ability to read in their second language. However, when students are older and they are illiterate in their own language how can they be literate in English? Therefore, in this situation it is true, students transfer their first language skills to English. They can compare their native words and English words, compare grammar differences. It should help. When students are a little older, it is easier to compare your native language system to the system of the language you learn. It is that age when you look for some aspects in the foreign/second language that you can compare to your mother tongue.