That's a
good point. Nevertheless, it's still a short term
consideration - purchasing power and exchange rates will converge over
time (and then diverge again, of course, but there's no way to know
which direction they'll go in next time).
That's not a given. It depends
on getting the economic fundamentals
right at the national level, like not depending on debt to fuel
spending. It will work as a short term tactic, but cannot be sustained
as a long term strategy. Exchange rates merely reflect purchasing
power, and that in turn is a composite of a number of factors. Some
countries have gone bankrupt in the past, and the old currency became
worthless.
The modern world economy is based almost 100% on debt. Pretty much
every unit of currency in existence exists because someone lent it to
someone else (ie. all money is in the form of an IOU and is not backed
by anything - the gold standard went the way of the dodo years ago).
There is nothing wrong with debt, it's just unserviceable debt that's
a problem. The current economic difficulties were caused by banks in
the US giving mortgages to people that couldn't afford to pay them
back. If they'd been able to pay them back, there wouldn't have been a
problem.