What about the Pietrowsky ?

The Pietrowsky defensive 4...exf3 5.Nxf3 Nc6 (diagram) is difficult to play for both black and white.

White's careless 6.d5 is weak, as the pawn on d5 because an easy target after 6...Nb4, so white should definitely choose 6.Bb5.

Black's natural reply is 6...Bd7, preventing a doubled c pawn. White's strongest move is supposed to be 7.00, but 7.Qe2 is surely worth a try.

The position after 7.00 is doubled edged : White has some initiative for the pawn, but black has no obvious weaknesses, so white must be very carefull not to overextend.

a/ 7.00 e6 8.d5

...a1/ 8...Bc5+ 9.Kh1
......a1a/ 9...Nb4 10.dxe6 (=)
......a1b/ 9...exd5 10.Nxd5 (=)
......a1c/ 9...Nxd5 10.Nxd5 exd5 11.Qxd5 (+)
......a1d/ 9...Ne7 10.dxe6 (+=)
......a1e/ 9...Nb8 10.Bc4 (+=)
...a2/ 8...Nb4 9.dxe6
......a2a/ 9...fxe6 10.Bc4 (=)
......a2b/ 9...Bc5+ 10.Nd4 (=)
......a2c/ 9...Bxb5 10.Nxb5 (=)
...a3/ 8...exd5 9.Nxd5 (=)
...a4/ 8...Nxd5 9.Nxd5 exd5 10.Qxd5 (+=)

b/ 7.00 a6 8.Bxc6 Bxc6 9.Ne5

...b1/ 9...e6 10.Nxc6 bxc6 11.Qf3 (=)

So white's winning chances are quite limited after 7.00. Let's now look at the alternative 7.Qe2.

c/ 7.Qe2 a6 8.Bc4

...c1/ 8...e6 9.d5 Na5 10.dxe6
......c1a/ 10...fxe6 11.bxe6 (=)
......c1b/ 10...Bxe6 11.Bxe6 fxe6 12.Bg5 (+=)
...c2/ 8...b5 9.Bb3
......c2a/ 9...Na5 10.Ne5 (=)
......c2b/ 9...e6 10.d5 (=)
......c2c/ 9...Bg4 10.Be3 (=)
......c2c/ 9...Bf5 10.Ne5 (+=)
...c3/ 8...Bg4 9.Be3 (=)
...c4/ 8...Bf5 9.d5 (=)
...c5/ 8...Na5 9.Bd3
......c51/ 9...g6 10.b4 Nc6 11.Ne4 (=)
......c52/ 9...Nc6 10.a3 (=)
......c53/ 9...Bg4 10.Ne4 (=)
......c54/ 9...e6 10.Bg5 (=)

d/ 7.Qe2 e6 8.d5

...d1/ 8...Nb4 9.dxe6 fxe6 10.Bg5
......d11/ 10...c6 11.000 (=)
......d12/ 10...Nc6 11.000 (+=)
......d13/ 10...Bd6 11.Qxe6+ (+=)
......d14/ 10...Be7 11.Bxf6 (++)
......d15/ 10...Bxb5 11.Nxb5 (+=)
...d2/ 8...Nb4 9.dxe6 Bxb5 10.Nxb5 (+=)
...d3/ 8...Na5 9.dxe6 fxe6 10.Bf4 (+=)
...d4/ 8...Ne7 9.dxe6 fxe6 10.Bg5 (+=)
...d5/ 8...Nb8 9.dxe6 fxe6 10.Bg5 (+=)

e/ 7.Qe2 g6 8.d5
...e1/ 8...Nb4 9.Bg5 (=)
...e2/ 8...Na5 9.Bxd7+ (+)
...e3/ 8...Nb8 9.Bg5 (=)


So there is no spectacular chess after black's timid 5...Nc6. Playing solid seems to be the recommended approach. The problem is that there is not much else after 5...Nc6. 6.Bd3 is obviously bad because of 6...g6 and 6.Bc4 is countered by 6...e6.

So the only alternative to 6.Bb5 is moving the queen's bishop, either to e3, f4 or g5.

Anyway, seems like the Pietrowsky is a simple equalizing line for the cautious black defender.