I have the Swarovski doubler for my 10x42's. Seemed like a good idea since I use my 42's off the tripod a lot, however, it is useless. FOV is terrible and image quality is compromised and low light is terrible. I find the doubler more useful as quick reference glass from the pocket then as an attachment to my eyepiece. I would definately recommend you do not spend the money. In my case, the swaro attachment just to mount on the binos on the tripod was a hundred bucks+, throw in the actual cost of the doubler and you are paying a fair amount for a marginal system. Swarovski makes great stuff, and I love my binos, but the double system just doesn't work.
Gotta agree with Two Bear. I have the exact same setup and rarely use it for the same reasons. Too expensive for what you end up with, and it also compromises the binoculars (won't fit in my Badlands case, ties them up for quick useage, and switching out in the field invites dirt, moisture and fumblage). Bite the bullet and take a good spotting scope with you. The Swarovski spotter is awesome (but heavy and big). Haven't used one myself, but Mike Eastman swears by the Nikon ED 50 as a smaller, lighter alternative.