Inspired by Knock-on's comment about Schumacher being one of the worst performing team-mates over the last 3 years, I decided to look up the championship points tables and work out just who have been the tardiest team-mates in that period. Each line shows the name of the driver, the team-mate they were beaten by, their team and their championship points score as a % of their team-mate's score.
Italics indicate scores that are less statistically valid because both drivers had 2 or fewer points scores, or both scored less than 10 points in total. And the unquantifiable bottom 3 teams are omitted.

2010:
Code:
1 Petrov      (Kubica, Renault)       20%
2 De La Rosa  (Kobayashi, Sauber)     29%
3 Liuzzi      (Sutil, Force India)    45%
4 Hulkenberg  (Barrichello, Williams) 47%
5 Schumacher  (Rosberg, Mercedes)     51%
2011:
Code:
1 De La Rosa  (Kobayashi, Sauber)         0%
2 Maldonado   (Barrichello, Williams)    25%
3 Senna       (Petrov, Renault)          40%
4 Massa       (Alonso, Ferrari)          46%
5 Buemi       (Alguersuari, Torro Rosso) 58%
2012 (so far):
Code:
1 D'Ambrosio  (Raikkonen, Renault)  0%
2 Massa       (Alonso, Ferrari)    36%
3 Schumacher  (Rosberg, Mercedes)  46%
4 Grosjean    (Raikkonen, Renault) 56%
5 Webber      (Vettel, Red Bull)   71%
Schumacher does indeed make the top 5 worst team-mates in 2 years out of 3. Statistically Massa looks like he comes out even worse: if you ignore the "statistically invalid" entries in 2011 he's topping the table. However you might consider Alonso to be a more testing standard to measure against than Rosberg.
In 2012, Grosjean's score is surprisingly decent for a guy who crashes or gets banned every other race.

Finally looking at the top 4 teams, who have all had stable driver pairings, over the 3 years:
Code:
1 Massa       (Alonso, Ferrari)   47%
2 Schumacher  (Rosberg, Mercedes) 59%
3 Webber      (Vettel, Red Bull)  76%
4 Button      (Hamilton, McLaren) 99%