Berlin area is much poorer than the rest of Germany, why not give Berlin a separate currency too?
If you'll read up on currency theory, you'll find some people have proposed pretty much exactly this (Jane Jacobs comes to mind, probably in Cities and the Wealth of Nations, it's been a few decades since I've last read it).
More generally: with a sovereign currency, a country controls both its monetary and fiscal policy, and can make its own allocation decisions with both. Which means it can set monetary policy to benefit its industrial / manufacturing or other economic centers, and use fiscal policy to create wealth transfers to underdeveloped regions. This is effectively what happens in the United States, for example, through SSI, welfare, medicare, SNAP, and similar programs.
In the EU, you have a bifurcation of monetary policy (ECB) and fiscal policy (independent European states). This was pointed out by numerous early critics of the EMU, including rather notably Margaret Thatcher, in the late 1970s / early 1980s. Her comments then proved exceptionally prophetic.
If you'll read up on currency theory, you'll find some people have proposed pretty much exactly this (Jane Jacobs comes to mind, probably in Cities and the Wealth of Nations, it's been a few decades since I've last read it).
More generally: with a sovereign currency, a country controls both its monetary and fiscal policy, and can make its own allocation decisions with both. Which means it can set monetary policy to benefit its industrial / manufacturing or other economic centers, and use fiscal policy to create wealth transfers to underdeveloped regions. This is effectively what happens in the United States, for example, through SSI, welfare, medicare, SNAP, and similar programs.
In the EU, you have a bifurcation of monetary policy (ECB) and fiscal policy (independent European states). This was pointed out by numerous early critics of the EMU, including rather notably Margaret Thatcher, in the late 1970s / early 1980s. Her comments then proved exceptionally prophetic.