Re: Because the Pegasus worked out so well for Orbital.
The only advantage I can think of is that it doesn't require a large flat open bit
The other advantage is that it can fly to the equator, launch there and come back without paying the French. Same as SeaLaunch - that alone is ~ 5%.
Savings for actual high altitude launch are not so shabby either. In reality you use ~ 30% of stage 1 fuel to reach Mach 1. 50% to reach mach 2 and stage one on most rockets is fully spent and separates at Mach 4.5 or thereabouts. The speeds and launch profiles for most orbital vehicles are well known - have a look yourself (these are the numbers for Soyuz).
Thus, a launch using a Boeing flying to the equator, launching at maximum safe speed (about Mach 0.8) and maximum altitude (about 13km) is ~ 25% of stage one fuel. If you launch from something that can actually carry a sufficiently large payload things start to look interesting.
Here is why the Bearded idea is actually daft. While Boeing 747 can theoretically carry significantly more than the ancient 1011L used in the Pegasus program (70 tons vs 23 tons), it cannot carry it on the wing mount. The size there is determined by the wing structural strength and is probably going to be under 15 tons.
While at it - the 70 tons you can top-load on a Boeing 747 is still very short of the 300 tons of a Soyuz launcher. However, when you compute all cost/fuel savings it is equivalent to a 100 ton launcher which is not to shabby. You can throw some pretty good sized payloads into space with that. However, for that, the Bearded one will have to do what NASA did with its B747 shuttle carriers - modify it into a proper launch platform. I doubt he has the resources (and the motivation) to do that.