Two extruders?

I know many people are doing two extruders nowadays, and I am not quite seeing why?

I know you can use two colors, but the slicing software does not seem sophisticated enough to really do two colors on outer edges at different spots as far as I can tell.

It does not seem that it would really make for much faster printing.

I just don't get it, can anyone help me understand the advantages?

Supports that dissolve in water is one good reason.

I kind of want to try mixing PLA or ABS with Nylon to have a part that is rigid in some parts but flexible in others.

I'm in the process of building such a machine. I was going to use the second head for PVA support material but I can't get anyone to sell some to me at a reasonable price. So, in the mean time, I was going to put a 0.5mm J head on the second extruder and use it for infill. If i put a 0.35mm into the first head and using the multi extruder features of slic3r I think I can take a big chunk out of my print times without loosing quality.

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Using ABSPrusa Mendel Zaphod with Pronterface and slic3r 1.3.0. Printing well with 3mm PLA and ABS through 2 x J Head Mk IV b and Wade Geared Exruders. Controlled using RAMPS1.4 board running Marlin_v1.1.9

I just realized that you should be able to use ABS as a support material for printing polycarbonate. ABS dissolves in acetone, whereas PC doesn't. To be accurate, PC does start to look a bit sick if you leave it overnight in acetone, but it doesn't really dissolve like ABS does. And hey, Nylon is acetone-proof as well, so ABS should work as support for that too.