In light twins (355s, 135s, 109s) and singles single pilot operation is not only perfectly acceptable but usually sensible, providing that pilot is well qualified, experienced and competent. There is surely rarely any situation in which flying workload makes a second pilot highly desirable, but what is certain is that the extra 85kg odd of pilot will always reduce some or all of payload available, climb performance, cruise speed and particularly range. Given an IFR trip I'd usually take fuel over a second pilot, for sure! But there again, I do accept the company can be nice! Of course medium twins are obviously different with a P2 having less influence on all the above.