The main arguement against using solvents/degreasers is that they are designed to first disolve the old lubricant so that it loses most of it's viscosity (and adhesive properties) then evaporate, leaving things not only clean but dry (as such completely unlubricated). If you can get at all surfaces that need to be lubed after cleaning this is fine (just relube the surface). The friction surfaces of a chain though are difficult or impossible to get at directly (without breaking the chain at every link). As such, the inaccesible friction surfaces are only cleaned by flushing and dilution.
Using a typical solvent/degreaser on a chain will generally leave some solvent residues in the most inaccessible places (on the pins under the rollers) because these areas are designed to be difficult to penetrate. After relubrication, these residues will tend to break down the newly applied lubricant and reduce it's effectiveness.
If one still feels that it is necessary to degrease a chain, two possible recommendations:
-If using a true solvent, after cleaning go through and dry the chain thoroughly using high pressure air which will have a better chance of removing solvent residues.
-Rather than using a true solvent, degrease the chain using kerosene which, even if it leaves residues won't actually destroy the viscosity of new lubricant when it is applied (although it will reduce it).
Degreaser is fine to use on chain rings and cassette (although not on derailleur pulleys, see above) as long as the chain is off. One does need to be careful with composite frames and wheels though, to keep any solvents off the composite material, since they can degrade some of these materials. I prefer just soapy water and a toothbrush (if not terribly gunky) or a wire brush (if really gunked up).
Bike shops use degreasers because cleaning goes faster and the results 'look better' for their clients. They, of course aren't necessarily interested in getting the ultimate longevity out of the equipment.
"I just want to thank everyone who made this day necessary." -Yogi Berra
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