When/if you defeat Miraak, you'll get the souls that he stole back.
So killing dragons and collecting souls is pretty useful beyond just activating Shouts, but not by much. You really want to visit the graybeards and ask if they sense any more words in Skyrim. They'll mark your map with a location that includes wall with a shout. Once you have both the shout and the soul, you're in business! Gathering words without dragon souls with which to activate them isn't very good and storing lots of dragon souls without shouts to activate is equally futile.
Looks like you've got your work cut out for you in Solstheim! Good luck, that's a tough place.