When I seized my top cylinder it was a mess. The piston chewed right through the nikasil and cracked basically every port. I had them both repaired in brisbane and the bike is like new (with a few mods).

Electrosil are way better with repairs and even have people buy new cylinders then have the nikasil stripped then replated with electrosil. So that says it all for me.

And I don't believe having spare cylinders makes a difference if your bike is jetted properly as it shouldn't fail at all, only wear with time as all parts do.