Ok while its right to say the SLR's engine is in front of the
passenger compartment, I think one thing most people here have
missed out, is that its mounted behind the front axle.
That in effect, makes the SLR a mid mounted engine car. an MR. mid
mounted is defined by having the engine anywhere between the 2
axles. Doesn't say where the passenger compartment has to be. Thats
the reason why the SLR has such a long bonnet.
For most exotics, the primary design concern is how well they
drive, which is why most of them are mid-mounted. Secondary
concerns like luggage space are often given lower priority. Most
rich people who buy cars like these, have luxurious estates for
longer get-aways, so there is essentially no need to pack in
luggage space into exotics. Also, packing in empty space for
luggage interferes with the packaging of the car, esp with
aerodynamics and mechanicals. These cars are focused on
performance, and are designed as such.
Mercedes cars tend to be front engined, not just the SL. Its not
exactly a tradition though. As we can see, the SLR has gone for the
MR route.