How much is traditional grammar effective especially in present era when more attention is focused on functional grammar?
I am not sure what you mean by the terms you have used.
If, by traditional grammar, you mean the rules I learnt when I was at school in the 1950s, then those speakers of British English who care about such things do not feel in any way obliged to observe what they consider to be artificial rules. My impression is that speakers of American English attach more importance to these rules.
If, by functional grammar, you mean descriptions of what has been observed, then most (semi-)educated speakers of British English accept and use what is generally accepted and used.