I have a commercial fixture that takes a metal hallide bulb, a transformer and a starter. What would happen if I were to wire direct the two wires that come into the fixture and bypass the transformer and starter and just use a regular light bulb. What is the benefit of using the metal hallide bulbs?

... It would work but you have voided the UL listing so
An Inspector wouldn't like it or an insurance adjuster!

Metal Halide fixtures are widely used for the amount of lumens per watt
They produce and the estimated life is 20,000 hours compared to incandescent
Which is 2,000 hours to as little as 750 hours.

Besides... have you ever seen a 400 watt incandescent lamp? ITS HUGE!