It depends on the team. Alonso and Mascherano hardly scored a lot did they? Yet the team we had with them was still challenging for the league and the Champions League. Having two players in there like that isn't a problem as long as they're good enough, the team's set up to make use of them and we have other players who can score. It's not Lucas' fault if our front players can't score and if we use Henderson and Lucas together, then we need to make sure that we're set up to make use of their strengths and we can get goals from other areas. That's not to say Henderson doesn't need to improve or that we couldn't do with Aquilani or that the front players haven't been good enough etc - it's not a necessity to have players in CM who score a lot of goals.
Edit: sangira's post as well is spot on.
Missing the point by a country mile here.
Mascherano and Alonso did other things that
enabled the team to score a shit load of goals. Mascherano could win the ball any fucking where on the pitch and give it to teh more gifted players , Alonso could pick out any player anywhere from any position , while simultaneously dragging two players out of position. They had well defined roles in the team and carried it out to perfection. They were absolutely crucial to the way we played.
While all Henderson does is get the ball , pass it to another player instantly , usually a 5-10 yard pass sideways or backwards , run around aimlessly chasing the ball to no great effect and make the occasional forward run or ping in a cross , again usually to no advantage to the team.
As far as Sangria's post is concerned , I would like him to expound a bit more.