Because you see in the last years that seldomly one of the top 3 won Paris. They always pull out and rest for the TMC or are injured after a long season. Maybe Federer will rest for London, or just lick his wounds after a bad loss yesterday. He's done it before.
Murray is so injury-prone he should really rest if he wants a real shot in London, in front of his home crowd. These guys can't really care too much about another Masters, the TMC is much more important for them.
I think the situation is just a bit different this year than recent years. I don't see Federer pulling out at all, as the #1 YER is still at stake - it's NOT a done deal yet (
), and Nadal will surely force him to work hard on sealing the deal.
And that loss yesterday wasn't as bad as some make it out to be I think.
Murray is coming back from an injury, did well in Valencia, and may well want to improve his match rhythm in order to be able to do well at the WTF. So I don't see him pulling out either. Djokovic is in good form, doing very well right now, I'd think he'd like to continue on his good winning mood right now.
And of course, there's still a spot for the WTF at grabs - all possible candidates will try their best to grab it.
I wouldn't be surprised if this would become the best Paris-Bercy tournament we've watched in years!