The argument that things should be getting tougher as the years go by is fully logical, but up to now it's not what has happened. Is this because he is still rediscovering his speed? When will this stop? It's not apparent that it has done so until now, so I wouldn't use this criterion for evaluation purposes. Additionally, I really wouldn't go into the debate about how much Schumacher costs Mercedes, as we don't know precise figures, in particular in regards to how much revenue and exposure he provides for the team.
So the main aspect to look at would be his performance level. If my opinion were asked and valued, I would say that 2010 was a big disappointment, particularly until its last quarter. Since then, Schumacher has been at Rosberg's speed in the races, but duly outqualified throughout 2011. Also, he committed to a fair number of uninspired moves last year which, coupled with reliability related DNFs set him behind Rosberg in the standing, but I'd really say they were evenly matched in 2011, as far as race speed is concerned.
2012 seems to be a completely different experience. I can't say yet if Rosberg still has an edge in qualifying or not, because it's not at all clear to me. Just based on results, they are 5-4 in Rosberg's favor, but Schumacher had a DRS failure in Bahrain, he didn't run in Q3 in Spain and he failed to set a proper time in Q3 in Canada. Also, it's worth noting that until yesterday, Rosberg was the best qualifier on the grid, as far as the average starting position is concerned.
In races, Schumacher finished four out of nine. In those four, he finished ahead of Rosberg three times, and when he didn't it was because he had to start from 22nd after the aforementioned DRS failure. In Australia he was considerably ahead when his car failed and in Monaco he would've been ideally positioned to win. So I'd say he was arguably better than Rosberg in most races, thanks first of all to his improved qualifying form.
There are two questions which I assume he will consider, beyond the foregone assumption that he still wants to race. The first is whether he can be consistently fast, which we've not been truly able to gauge due to the many problems he's had this year. The second is whether it makes any sense to continue. Can Mercedes offer a car that is reliably fast and which allows him to at least challenge for podiums on his own strength? There's nothing to be gained in outdriving anyone if you end up 6th-7th-8th.
I reckon that if things go reasonably well in the next three races and the decision is to be made within six weeks, he will want to stay on another year to see the fruit of their labor - i.e. what the combined efforts of Bob Bell and Aldo Costa can bring to the car.
