Every cigar smoke starts with the cut and every drag you take is affected by it. Hence it must be perfect.
Using double blades ensures a precise clean cut. When using a single blade, half of the cigar is unsupported as you cut causing it to rip. This leads to having to spit or swallow tobacco flakes as you’re smoking. Even a slight rip can seriously damage the drag. The draw will be loose and cause too much air to pass through as you smoke. Your cigar will then overheat and have a more bitter taste, which is not desirable.
With sharp double blades, this isn’t a problem, as pressure is coming from both sides cutting the wrapper leaf cleanly. Use a razor-sharp, solid, high-quality cutter.