MOTM

a) the resistor is used to minimize DC offset drift over temerature.

b) the 'bad effect' is added noise, but in this use it doesn't matter. It 
also can add additional DC offset but the 1V/Oct trimmer "compensates" for 
that :)

The LT1013 has relatively high input bias/input offset currents, but very 
low offset *drift* (it was originally designed as a thermocouple amplifier, 
where in a 'previous life' I used it for pizza oven monitoring). We really 
don't care what the final DC offset is, we just don't want it to *change* 
over temperature. The LT1013 is one of the major reasons MOTM products are 
so temperature stable.

Paul S.