The general rule of thumb in the Philidor Counter Gambit is that it is unwise for either player to grab material and hold onto it, if it means ceding the initiative.
Before analyzing Barker's line, I would like to improve on Dennis Monokroussos's 9.Bd2 Bxd2+ 10.Qxd2 Nf6 11.O-O-O O-O 12.Nxd4 Qd6 13.Rhe1 g6 14.g3 Nh5 15.Re3 Ng7 16.Rde1 Na6 17.Re7 Nc5?!.
The black queen knight is needed on the kingside. Correct for Black is 17...Nc7 18.Qc3 Nce8 19.f4 Nf6 with a tenable position.
After Barker's 9.c3!?, there follows 9...Qe7+ 10.Kd2 dxc3+ 11.bxc3 Qf6 12.Re1+ Ne7 13.Qc2, as given by Monokroussos
In keeping with the afore-mentioned rule of thumb, Black should forgo Monokroussos's 13...h6?! in favor of 13...Bd6 14.Nxh7 Rxh7!? 15.Bxh7 Kf8 16.Kd1 Bg4 17.Qe2 Nd7 18.Bg5 Bxf3 19.Qxf3 Qxf3+ 20.gxf3 Kf7 21.h4 Rh8 22.Bc2 Nc5 23.Rb1 b5.
Although down a full exchange, Black will have counterplay against White's shattered pawns. I like the plan of 24...Nc8, 25...Nb6, and 26...Nc4.
