It is far from a meaningless metric.
The more transactions there are, the more exchange is there within the economy.
This is a fallacy. Transactions simply represent money moving between addresses. They do not represent money moving between people. And in particular they do not imply trades of bitcoins for goods or services. I can shift money back and forth between two addresses all day long and pump up the transaction count, but I wouldn't be doing anything for the Bitcoin economy.The more transactions there are, the more exchange is there within the economy.
If you believe this metric is meaningless, what would you say if the number of transactions lessened by 90%? Would you say that nothing has changed? The economy is fine?
I wouldn't say nothing has changed. I'd say that the number of transactions has decreased by 90%. That's all I really could say. It might be the case that less trade was occurring using bitcoins. However, it might instead be the case that more trade was occurring using bitcoins and it was simply being represented using fewer transactions (i.e., more efficiently). Neither the number of transactions nor the sum of the transaction amounts tells me anything conclusive about the amount of goods and services that are being traded for bitcoins, which is the metric I would use to represent the health of the Bitcoin economy. 
