A formula in a Workflow should be able to calculate with different currencies.

Use case: we have opportunities in different currencies. In order to calculate expected contribution margin, we link a product to the opportunity that has a cost in USD (currency field).

A Formula in a Workflow is used to calculate the contribution:
contribution = volume * (price - OpportunityProduct.cost)

for a EUR opportunity, this goes wrong: the formula just takes the number without taking the exchange rate into account.

We use a workaround: user fields for exchange rates (eg EURtoUSD) are set to a fixed value with a formula. The workflow for the contribution then checks the currency of the opportunity (CASE) and uses the user fields to convert the product cost.
=> I would think that this should be done automatically.