Inboarded shocks have the benfit of being easy to install, and also give you more wheel travel than the shock would allow if mounted vertically. A 12" shock could probably give you 16" of wheel travel with an aggressive inboarding setup.
Down sides are that the shock doesn't have the same damping rates anymore. If you imagine that a shock has 100% damping effectiveness when mounted vertically and 0% effectiveness when horizontal (duh!) then you can extrapolate almost any other value in between. The set-up pictures (which looks like about 45* angle) would therefore provide 50% of the normal damping.
You will have less control on sidehills with an inboarded setup also....