Thought I'd put these two cents in:
Your LBS' excuse is both ridiculous, and indirectly funny (read=stupid), for a longer chainstay length would actually improve chainline by reducing the Y-axis offset of the chain.
I dare say that a recumbent (ducks for tomatoes), for instance, will have a slightly better chainline then most VLWs, and, let's say, a 10-speed musclebike with 20" wheels and very short chainstays will have chronic front derailer/chain contact. (This is probably why you haven't seen 14 or 21 speed BMX bikes at X-Mart, or anywhere else for that matter.)
Of course, the slop in the chain of the recumbent might still make a racket. They ought to design a special derailer for recumbents with an exceptionally strong pulley cage spring. Heck, this might help to alliviate chain-skip problems on Shimano Hyperglide freewheels...
Take care,
-Kurt
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