I've mentioned this before but I just had a great example of why this matter so much I thought I'd share it again.

My friend and client Crys  needed to get home from Mallorca and wanted to use miles. They have Alaska Miles, Delta Miles and Chase Ultimate Rewards points. Getting home from Mallorca proved challening in general but flights to Barcelona are about $25 so we turned our search from Barcelona. Using Alaska miles it would have been 32.5K miles and $257 each with a stop in London and San Francisco. Delta wanted 66K miles each (yes this is coach). So I did a magic search and found Air France had availability Barcelona-Paris-Seattle for 20K and $125 each. Score!

Crys doesn't have Air France miles. But, Chase allows you to transfer Ultimate Rewards points Air France Flying Blue on a 1:1 basis. She quickly transferred 40K miles and within a minute they were in her Air France account and the flight was booked!

Chase, Amex, Citi and Capital One all have transferable currencies (with the right cards) and can make the difference between 20K miles and 66K for the same booking!