Guys - in JAA land (which is different from the USA [FAA] land) we have two distinct components to the "pilot certificate".
The first bit is the LICENCE - you are either licensed to fly privately (PPL) or professionally (CPL/ATPL).
The second bit are the RATINGS that are attached to that licence. The ratings cover aircraft classification (single engine, multi engine, turbine engines, instrument flying, night flying - whatever) - which is why there is a difference in SE and ME instrument rating privilidges.
The LICENCE dictates whether you may fly for pleasure or professionally. You are then able to fly, either privately or professionally, in whatever type of aircraft or weather you are RATED in - so having a multi engine piston RATING, fo example, means you may only fly it privately if attached to a PPL or either privately or professionally if attached to a CPL/ATPL.
If you buy yourself a Boeing747 and have a B747 type RATING attached to a PPL - then, no matter how many thousands of hours you may log whilst flying your B747 - you can only fly one privately. You cannot fly one professionally - so no airline job.
So - PPL on a SE aircraft - you fly privately on SE aircraft only. If you add a multi engine RATING to that PPL then you can fly either single or twin engine aircraft PRIVATELY.
If you go on to get a professional licence - then you can fly whatever types/classes/weather that you are RATED for, professionally.
If you have a PPL with both SEP and MEP ratings then there is little value, one way or the other, if you do the test in either a single or a multi.
Why would you do the original CPL in a twin? Because it's much cheaper and it's, arguably, much easier.
REMEMBER - all the above is for JAA, not FAA.