I guess the hope is that the 2+ years in a new / different environment alongside World Class players (using Jack at Toulouse as an example) will improve the player to the extent that he (or she, considered in the wider picture) is able to bring that new level to the national side.
Younger guys like Pearson, CCS etc will put pressure on, but look at the clamour for Mercer to be brought in pre-RWC given his displays in the Top 14 for Montpellier.
You could argue a case that absence makes the heart grow fonder, and that Mercer (for example) is more visible on Top 14 highlight reels than if we saw him week in / week out in the Prem.
I'm firmly in the camp of letting players play wherever, and picking them anyway. Pick your best players, regardless of their club, a la football. Leads to an exodus of "top talent" from the Prem? Opens up opportunities for more youngsters to break through and stake claims.