That automatic renegotiation is useful for poorer countries though. If a country exports a lot more than it imports then the valuation of their currency increases, meaning that buying from that country becomes more expensive (because you buy with their currency and there's more demand for the currency). This enables poorer countries to step in and offer to export similar goods for cheaper. It also means that poorer countries, where their imports exceed their exports will get more investments, because their currency is worth less.
A simplified example of the above:
Let's say that Germany kept the Deutsche Mark (DM) and Italy kept the Lira. Let's assume in this case that Germany's exports exceed their imports (trade surplus) and Italy's imports exceed exports (trade deficit).
In the case of Germany, because their exports exceed imports, the value of the DM would go up, because there's more demand for the DM. This would make goods from Germany more expensive (your dollars can buy fewer DMs) and it would slightly diminish the exports of Germany. Due to the DM being more expensive it also means that foreign investment in Germany is more expensive, because your dollars can buy you fewer DMs.
In the case of Italy, because their imports exceed their exports, the value of the Lira would fall, because there's less demand for it. This means that goods from Italy become cheaper (your dollars can buy more Lira) and it would increase Italy's exports. Due to the Lira being less expensive it's cheaper for foreign investors to invest on Italy, because you dollars can buy you more Lira.
When both of these countries adopt the euro, you will have to add up their exports and their imports to find out whether there's a trade surplus or deficit. Let's say that when you add them up and compare you find that you have neither a trade surplus or deficit. This means that goods exported by Germany won't increase in price and the goods exported by Italy won't decrease in price. This is advantageous to Germany and disadvantageous to Italy. It also means that Italy won't gain an advantage in attracting foreign investments compared to Germany either (based on the currency).
In reality, the topic is much more complex, but different currencies seem to have a balancing effect on exports, imports, and foreign investments. Strong exporters in the eurozone benefit from the countries that import a lot.