From Sheldon Brown
An idler is a pulley or roller that does not produce any mechanical advantage, nor transmit power to a shaft. Idlers are used to lead a chain around a bend (as in short wheel base recumbents), or to take up slack in a drive chain (as in a tandem chain tensioner, or the pulleys in a derailer "
Like on a fixed gear bicycle, the timing chain on a tandem must remain fairly tight all the way around. If there is much slack, the chain would fall off. In cheaper, homebuilt, or older tandems, an idler pulley is an easy way to achieve the desired tension. The other option, which most quality tandems have, is an eccentric bottom bracket. This eccentric allows the bottom bracket to be rotated to tighten the chain tension.
Adding on, the idler pulley is simply a different and less expensive way to manage timing chain slack and, from a technical standpoint, works just as well as an eccentric bottom bracket.
IMHO, the idler pulley has a few positives going for it:
1. They work without altering the saddle set-back and height in relationship to the front crank axle that occurs over time as an eccentric is rotated to take up chain slack.
2. The sync chain adjustment and removal is very quick and easy
3. They are less costly and typically weigh less than the added weight of an eccentric / eccentric shell.
4. When properly designed and adjusted they work just fine... noting that every multi-speed bicycle with a rear derailleur uses an idler pulley.
The downsides to idler pulleys include but are not limited to:
1. If they are poorly designed or mis-adjusted chain drops and derails are common, annoying and can create a hazard.
2. They are easy to bend and damage.
3. They just miss the mark in the aesthetics department.
Thanks a lot I went by my LBS and asked this questions and got same answer.
It is good that the LBS understands idlers. If you had received a different answer, I would suspect an uninformed employee or shop and maybe somewhere you would not want to rely on their recommendations for your purchases.