They're not mutually exclusive, but since there's no way of knowing the thoughts that went into the transfer that seems like a more unlikely assumption.
It's a simple question of "what then would you expect to happen"? If a player is signed primarily for non-sporting reasons then I would expect that player to get less game time than a player that was signed for sporting reasons. That's won't always be true, but that seems more likely. Occam's razor and all that.
If Dempsey was a target for Liverpool for non-sporting reasons it makes little to no sense that they would refuse to pay the extra million in transfer fee that Fulham were asking.
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Slightly different approach:
We sign quite a few players each season. 6 last summer and Holtby in January, that's 7 under AVB in his first season. To me it seems perfectly natural that some of those signings will perform better than other just by random chance. Much like, perhaps partly by random chance (given that performance will be influenced by this), any given fan is likely to rate some of those 7 players highly and others not so highly.
There will always be some explanation available to describe why the player that doesn't perform or that isn't rated by an individual fan was signed. If it was Lloris that you didn't like could you claim that we were just trying to improve our standing in the French market by signing their international captain cheaply?
If it didn't work out with Vertonghen we could find some explanation that he was just signed because he was Belgian and on his last year of his contract. Ade was just signed because of the African market (we have after all had summer tours in South Africa)? I could go on for most the players we've signed I believe.
It seems to me that these kinds of explanations will always be available, again, just as a result of random chance. So I'm rather skeptical of those explanations. I thin in general both Levy and AVB are rational people, trying their very best to make well reasoned decisions to help the club forward. To sign a £6m 29 year old primarily for non-sporting reasons without the manager rating that player seems to me irrational.
Like I said, I could see the kind of t-shirt value in a player like Toda, also a younger player on low wages I think who was never going to cost the club much. Dempsey doesn't to me fit that description.
Friedel would actually fit that description much better, free agent, short term contract and all that. Would have been a perfect candidate, if not for the fact that he did so well for us last season.