Yardy goes for little runs, creates pressure and causes people to go after Swann at the other end which leads to wickets.
I'm not sure this statement is entirely one of fact.
In the 8 games Yardy and Swann have played together, Swann has taken more wickets, but has also been more economical and bowled fewer overs. Both players have slightly better strike-rates than in their greater careers, but only Swann enjoys improved economy.
On aggregate and in a case by case basis, Yardy does not go for fewer runs than Swann, though both are substantially more economical than the overall run rate of the innings they bowled in.
In 4 games, Swann took more wickets, while also in 4 games, he went for fewer runs. 3 of these matches are the same; thus in 3 games he took more wickets
and went for fewer runs than Yardy.
In 3 games, Yardy bowled more overs. In two of these, Yardy went for fewer runs, while Swann took no wickets. However, in the third game, Swann took 1/31 in six overs, while Yardy was given three more to get 0/49.
With regard to Yardy contributing to Swann's wickets, there may be something to it. 46% of Swann's wickets have been taken after an over from Yardy, but these overs come with an economy of 5.33. On the other hand, 71% of Yardy's wickets have come after an over of Swann, whose overs brought an economy of 4.6.
tl;dr: It seems Yardy feeds off Swann and not the other way around. I think the best argument is that Yardy is competent enough and you could definitely get worse (eg, Bresnan). At such proximity to the World Cup, it may be a poor move to change something which does not seem to be overly consequential.
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