Hey Alexban,
Don't think this is the main reason. In case of a bleed trip off you leave the eng. bleed 'switch' in the on position. Only the affected pack is switched off. This opens the isolation valve aswell (in auto).
Actually there is not one situation (in the QRH) were an eng. bleed 'switch' is placed to off, with the isolation valve switch left in auto.
It is not always possible to avoid icing conditions, in this case wing anti-ice could still be required. Above fl 350 wing anti-ice could also cause a bleed trip off with both bleeds working.
Coming back to the question of Cakov. IMHO the function of the eng. bleed 'switch' position in relation to the isol. valve (in auto) is just a back up function. In case an eng. bleed switch is for what ever reason in the wrong postion during moderate icing, then at least both wings are de-iced. Further you could use this function, on the ground, as a handy tool (several possibilities to control the isol. valve).
Once again this is all theory. Nothing found in the books about this.