I've been discussing this topic with a number of colleagues. Put quite simply, the general view is upon being released to the line, a copilot with 1500 hours of SPIFR on a Navajo or similar will be far more use to a commander than a cadet pilot with minimal IFR experience. As much as anything else, the Navajo pilot will have experienced in copying down clearances, negotiating with ATC, slots etc. He will also have experience of running a commercial operation with commercial pressures that come with it, such as practical considerations to consider when planning to fly right down to Wx minimums, juggling the figures to make the loadsheet legal, etc. His handing skills will be more developed too, although this is an area were 'bad' habits can creep in.
I think these comments sum up this thread rather well, because without a decent basis of experience, the 250hr wonder is a liability rather than a contributer for those first 300-400hrs and effectively reduces the flight to a single pilot operation.