Even when I've done asymmetric biwire, it's still been better than single wire. Just don't go TOO far off design principles or lengths between the two. For a while I used a 6 ft AQ Mont Blanc (12 AWG copper) on bottom and a 7 ft KE-4 (15 AWG silver) on top and it sounded great, competing on par with the more expensive AQ Redwood symmetric biwire.
To those who say absolute symmetry is required - that "perfectly symmetric" signal is going through two TOTALLY different sets of drivers and crossover parts anyways.
I've moved on to symmetric biwire and its great, but like I said I'll still take most asymmetric biwires over single wire.