Allen has been getting himself into goalscoring positions throughout all pre-season and he scored a couple of important goals last season. I have no doubt he will create or score some goals if he gets an extended run in the team.
Coutinho of course offers more direct threat, but in the end it's about the balance in the team as a whole. Suppose you have Suarez back and he forms a good productive partnership with either Aspas or Moses. So with Sturridge cementing his place at #9, you now have 3 players with direct goal threat and enough skill and guile to make things happen - all they need is a consistent supply. In this case, Coutinho better stop giving the ball away like he does now - otherwise it will be very easy to make a case that a player like Allen is better suited to complement our attacking 3.
Look at last season, how many twists and turns there were in terms of personnel? Sterling and Suso were starting 11 players for a stretch, Wisdom played at RB and Johnson at LB, Downing was frozen out of the team and laughed at, then used as a LB and laughed at, then he became our starting RW for the rest of the season. Henderson looked completely devoid of confidence in the first half of the season and fans were giving him all kind of abuse (remember what people were saying when he missed that 1 on 1 chance in Europa league?), then he proved himself more useful than the ailing Allen and became a crowd favorite. Carra started on the bench and finished as an almost indispensable player, Enrique seemed like he was on the way out, then he was tried in midfield, now he is firmly established as a starter. Even Jones displaced Reina for a few starts.
The team is a living evolving organism and the only thing that's certain is that by the end of the season it will look very different than in the beginning. That Allen didn't play in the first 3 games means next to nothing. He is a good player, he is versatile and the manager knows his qualities. He will definitely have an important role to play this season and to think otherwise based on the first 3 games is just silly.