Almost invariably, moving the f-pawn in the first very few moves is not the best you can do and often almost deserves a (?). Openings like Bird's 1.f4, Dutch 1.d4,f5 are rare on top level, and the statistics shows why, and it should surprise nobody. As you say, it's weakening the king's position, and does not open diagonals that can be used for developing a piece.

But playable? Well, for most of us, sure. On the positive side, the pawn controls e5/e4. If it gets swapped there is an f-file to use for attack, or the f-rook can go to the third rank and further to the g- and h-file. At times the pawn is pushed further to attack.

When White plays 2. f4, this is exacerbated. Since he has already spent a tempo to push the f-pawn, the strongest defenses unsurprisingly call for an early ...d5. (This is why White often prefaces the f-pawn advance with 2. Nc3.)

I have often played the Grand Prix myself, and I can tell you that I felt relief whenever my opponent *didn't* play for that early break.

Part of the reason for this may lie in the disruption of White's attacking plan, which requires not an exchange on e5, but a push to f5 followed by an attack on the kingside with pawns, minor pieces and queen (the latter usually via Qe1-g3 or h4). But this works only if White can maintain a strong c2-d3-e4-f5 pawn chain to secure the kingside pressure. If for any reason he must abandon this idea, the Grand Prix player is left groping for a new plan.

Of course this doesn't mean the Grand Prix *can't* be played positionally, nor that White is doomed after ...d5. But the psychological disorientation caused by blowing apart the opening's raison d'etre cannot be discounted.