I built my slow-build -7 without a longeron yoke and honestly never saw a need for one. Your mileage may vary, but here were my experiences:
1) The longerons themselves can be squeezed with a regular 3 inch squeezer yoke by simply turning the squeezer around and squeezing from the "inside" of the fuselage. Of course once you get aft enough on the fuse that the squeezer won't fit between the two longerons, you switch to bucking.
2) Never really saw a need for another yoke. Seriously. I used my squeezer all the time and there may have been one or two rivets that I couldn't squeeze where a longeron yoke would have been handy. Out came the back-rivet set and I just back-riveted the thing. An adjustable set holder for your squeezer makes it quite the versatile tool.
Again, tools are always a good thing to have sitting there ready to be used, but my opinion is that I wasn't going to spend a couple hundred bucks (pun intended) on something that probably wasn't going to add a lot of value to my project.
FWIW: I'm in the minority on this issue. People swear by their longeron yokes, but I still don't get it.