Originally Posted by ppc
Because zero dollars can make the problem go away
I prefer not to spend my hard-earned cash on fixing things that are designed to work together in the first place.
It's zero dollars if your time and labor are worth zero, and if you actually get it working well enough that you never suffer equipment damage or sustain an injury from derailment. (I've heard lots of stories of both.)
As for the parts being designed to work together, even Shimano has put extended mounting tabs on many of their small chainrings--the sole purpose of which is to reduce the chance of the chain jamming between the small chainring and BB shell after the chain derails. Shimano has also patented several guides integral with their front derailleurs, and occasionally attempted to produce them, but all of them involved logistical problems which were far more easily solved by having a separate guide unit.
I do think the inner cage plates of front derailleurs have a remarkably good design these days, considering the many different chainring configurations they are expected to work with. But those plates are specifically designed for performing consistent and reliable upshifts. It is no indictment of their design that they are only accidentally semi-useful for preventing downshifts derailments because that function was not part of their design.
I do agree that there are some products which mask or sometimes even worsen a problem which is better fixed some other way. If your chain isn't releasing cleanly because your chainring teeth are badly hooked, adding a bash plate to knock the chain loose is a poor solution because it doesn't work very well and the impacts tend to increase the hooking problem. But an anti-derailment guide does no damage, can often prevent damage, and can even improve front derailleur downshift performance (since the inner limit screw setting no longer has to be a compromise between safe and fast downshifts). I'm sure there used to be purists who resisted having guide ramps under their brake blocks for easing wheel insertions too (ramps being a "patch" for bad wheel insertion technique) but sometimes, an easier way of doing things is simply an improvement on the previous "proper" way.
Nick
N-Gear