I never understood that choice either, maybe it is purely financial.
Federer doesn't need that
he's used to planning his schedules very carefully with Paganini and Luthi, I really don't think we're able to judge them
Practicing sessions are surely crucial for clay season : see last year it worked very well
He's also said that he wants to stay fit until the very-end of Wimbledon, and considering how badly he played in Wimbledon final last year, looking slow comparing to previous matches, I can perfectly understand that.
I surely think that the heaviest priority for him is to make a perfect Wimbledon : that's his
In RG, he can lose anyway even if he's in good form
Coming back in Roma might be a good choice if he plays well, as most of the top-players will have played previous two weeks in Monte-Carlo and Barcelona.
As for Estoril, it's a 28 draw, which means only 4 matches maximum.
If he doesn't reach final in Roma, it makes sense to play a little bit more before Madrid.
As for Monte-Carlo, an Italian website says that he got a WC :
For the second consecutive year requested and was granted a wild card for the ATP Monte Carlo.
but it's the only website saying that
I think it might make sense if he just makes two rounds here and exits, to get some practice like last year.
But if he goes far in the tournament, surely it changes his schedule a lot, and it would be hard to understand
I think the financial pressure is heavier for this one than for Estoril (see last year) as Rolex is the sponsor of the tournament
As for this WC, if the Italian website's information is not confirmed, I guess there will be some rumours until the end of the week