Roger Modjeski explains his reasoning behind this in the RM-10 manual. He refers to it as light loading. You essentially are increasing the damping. It also reduces distortion, and improves tube life as the output tubes do not have to work as hard. In the case of the RM-10 it will reduce power about 20% but given that there are 35 watts to start with (first 15 in Class A) the trade off is worth it and Roger recommends this approach with all his amps. The quote below is from Roger:
In addition to the RM-10 I tried it with a set of VAC Auricle Musicblocs and had similar results. So I would encourage anyone with a tube amp that has multiple taps to try it.
Basically, light loading reduces the output current demand on the output tubes, allowing them to be more linear. It also reduces noise, raises damping factor, reduces distortion by 78% and allows for 80% more peak current when needed. The only loss is about 20% of the power rating or 1dB.
In addition to the RM-10 I tried it with a set of VAC Auricle Musicblocs and had similar results. So I would encourage anyone with a tube amp that has multiple taps to try it.