For example;

Using your method, the sentence 'The big dog barked.' would be become 'ehT gib god dekrab.' This confuses the reader because they would begin reading the sentence from the left, but immediately find that they have to read each word from the right. The act of reading the sentence is no longer 'smooth' because the reader's brain is constantly having to switch between reading from left to right, and from right to left. Also the capitalisation of the word 'The' no longer makes sense when reversed to 'ehT'.

However, if you mirror the entire sentence, including punctuation, then it would become ".dekrab god gib ehT". If you then read the entire sentence from right to left you will find that you can progress from one end of the sentence to the other smoothly and without any confusion, punctuation and all.

It just takes a bit of lateral thinking, my man.

Shit, I sound like a teacher.