...but it's far less expensive than replacing your tandem with one that has more conventional sized seat tubes.

Seriously, shims and sleeves exist because they work well and, frankly, aren't all that noticeable. I have "normal" size seat tubes that take 27.2 seat posts which are too narrow for many of the stoker booms: only Cannondale's Coda-branded models are offered in 27.2. Therefore, I've had shims on our '98 Erickson from day one and haven't given it another thought until just now.

Bottom Line: If your stoker needs a shockpost find a good one and if it needs a sleeve/shim, so be it.