What is a disadvantage of decreasing the number of wire segments in an antenna model below 10 segments per half-wavelength?
Why Method-of-moments programs like NEC approximate the continuous current distribution on a wire by a series of short straight segments, each carrying an assumed simple current. Accuracy depends on having enough segments to follow the sinusoidal current taper, and the accepted rule of thumb is at least 10 segments per half wavelength. With too few segments the current near the source is poorly represented, and since feed point impedance is computed from the voltage and current right at the source segment, the calculated R and X can be significantly off.
Watch out Ground modeling is a separate part of the calculation (the ground model and conductivity/dielectric values you enter), not a function of wire segmentation; harmonic radiation and mechanical strength are physical antenna properties a model does not determine at all.
Remember 10 segments per half wave; too few segments, and the number you can't trust is the feed point impedance.