I reckon the heater pulls around 50 amps so that's quite a big draw on the batts before you even start to crank the engine. When my OE heater relay packed in (and was causing a permanent 300+ma draw flattening the batts when parked) I decided to I wanted to keep the heating facility so replaced the relay and wired a switch under the dash so I can enable/disable the heater at will. The benefit of the heater is it gives better and more complete combustion of the fuel on a cold start and will prevent bore wetting and oil dilution so I guess is aimed at engine longevity more than anything else, especially in countries with winters far more severe than we will ever see here. If the system is working correctly the time the heater stays on will vary according to engine temp. I just use the pre-heat when ambient temps get below freezing.
Once the engine has fired the heater will draw from both batts for the timed heating period but, as you point out, while cranking it's the poor old offside batt that has to supply the extra current which must have an effect on it's serviceable life compared the the nearside batt.
