I used to have pure silk base layer tops and long johns. Overall, they felt soft and nice. But the tops used to get smelly under the arms after one wearing, they felt clammy if they got sweaty, and they didn't keep me nearly as warm as merino wool thin base layers.
Wool gives you that lovely layer-of-air insulation that keeps you from feeling damp or sticky, and also wool doesn't get smelly because it does not encourage bacterial growth under your arms (like other fabrics do).