At the start of every turn, both players roll to see who goes first.
If the person who went second on one turn goes first on the next, they have an advantage that is potentially game-ending, and the other player basically has to just sit there and get wiped unless they play around the stupid rule by handicapping themselves by playing defensively.
It isn't THAT bad. People who complain about it are usually players of 8th edition, which - judging by everyting I know about it - was just as broken, just in different ways.
I have won games in which my opponent got the double turn, actually I think I even won more games without it than with it since I am a notoriously bad initiative roller it seems.
And yes there are ways of both mitigating the potential disadvantages tactically and to use it for your advantage if you get it at the right time. You can choose to gamble for it but you can also choose not to. Playing with the potential double turn in mind is not easy but worth it.
So I'd really disagree with this:
unless they play around the stupid rule by handicapping themselves by playing defensively
because it isn't playing around the rule, it is playing with the rule.
Now, I know there are armies that are devastating when they get the double turn but it is the fault of the armies. Those are armies like SCE that can deal mortal wounds on a horrendous scale, giving the opponent no means of protecting them against that damage, or massive shooting armies. They profit from it more than others because they are broken, not the double turn.
That being said: It is no problem at all to not play with the double turn and there are quite some people who play without it. Just ask your opponents, I'd probably shrug and say "OK, why not".