the plus is it's legal. why go to the bother of fitting/ re-fitting every MOT?
you'll prob find that most manuf. design engines & management systems to work as a whole unit.
In fact you'll probably find that a de-cat is illegal
. Its nothing more than a piece of pipe what goes in place of where the cat used to be. The upside is bit more top end power and torque. The downside is its very loud, has to be changed for MOTs, its Illegal, you lose some low down torque and sometimes it makes the Idle a bit lumpy.
A legal version is the so called sports cat, which is legal but at the same time allows less restricted flow. Downside to it is that its very expensive (about £250ish I think).
Most manufacturers design their engines and management systems to work as a whole but also are having to comply with all the rules and restrictions. Reason why they haven't fitted a decat as standard is because it would be too loud and the emissions would be too high. Same reason why air filters are usually enclosed types, rather than open air ones.
Hope it makes sense
Mig