Using clock and enable

I was given a code for a D Flip-Flop with enable.

process(clk, en)
    begin
        if rising_edge(clk) then
             if en = ‘1’ then
                 Q <= D;
        end if;
     end if;
end process;

1. I was told that I should not use if rising_edge(clk) and en = ‘1’ then .... Why?
   
2. Why isn't the if for the en = '1' before the if for clock since the clock is changing more often?
   
3. And is it necessary to specify that en in the process parenthesis process(clk, en)?