Well basically I already have this implemented on my site.. rather than taking in $_GET['currency'] .. my languages class files has all the currencies linked to their languages..
thus for example for Canada. I can show a Canadian flag icon and a Quebec Provincial flag icon.. the user can choose either and get the respective language but at the same time maintain the canadian currency.. as well I have a US flag icon and EU flag icon .. both are english language but have different currencies.. USD and EUR ..
currently I have all three currencies linked to one language directory English .. this is incorrect of course as there is US spelling and British/Candian spelling for certain words.. however these can be separated into different folders as well if wanted.
Benefit.. well the benefit I would say the user only has to click one link.. the language link which will automatically select the appropriate currency. Another benefit is sitemaps.. Google Sitemap Generator will create links for not only the currency but also the language.. so if you have 2 currencies and 2 languages.. you will get 4 links generated for each page.. if you have 3 currencies and 3 languages that would be 9 links for each page.. ridiculous isnt it.. well with the changes i made you get rid of the currency input.. thus .. if you have three languages.. only 3 links generated.. even if they are all in english.. it would be the same as if you had three currencies and only 1 language.. still 3 links..
If anyone is interested I can provide the changes..