Happy Feet

Well.. don't know if that makes me a better rider or not because I have no problem using it and seeing as I actually have experienced the set up I guess I know how it handles rather than imagining it. For off road with hill climbing, excess chain slap on a rigid mtb is not cool unless you like constant chain drop. Way easier to just avoid certain gear combos like high high which is quite easy considering.

It is a valid point that some shop workers can't conceive of anything outside the box and usually respond to such ideas as wrong or impossible for a few reasons. Some people probably never do more than look at and work on stock bikes and stock parts and don't experiment. Shimano says you can't run a 36T cassette on a XT derailer but it's actually common and I could even run a 40 on mine.

Old time shop workers also probably did not see the ratios that can be achieved today. If you focused on road bikes they usually had sprockets in the 11-28 range and double chainrings in the 50-30 range. Old school mtb's usually had stock clusters like 11-34 and triple chainrings in the 46-26 range. Even today the stock high range cassette is 11-36. A 40T cassette cog was not common nor was a 22 chainring. When would one ever have the chance to try a wide range scenario?

Today the clusters are edging 11-50, usually with single chainrings but 11-40 on compact doubles isn't a stretch at all. Sure, if you want to stick with stock or common ratios you will avoid extremes but people today in gravel and off road are pushing the boundaries of wide ratios for some pretty adaptable configurations.

Sapperc

Cyclists will do what they want, I certainly do. However, you should at least be aware of the possible consequences.

Shimano instructions for their derailleurs still warn against cross-chaining. Sram instructions actually advertise cross-chaining as a feature benefit. Not sure about others as I haven’t researched them.

That being said, I’ve never experienced any issues while cross-chaining with Shimano Ultegra 6800 on my Specialized Roubaix, old friction shifters Suntour on a vintage Nishiki International, or a vintage Trek 930 MTB with Shimano Tiagra.

I don’t usually cross-chain on purpose, but I’ve often lost track of what gear I’m in. It happens.

Cycle on!

ThermionicScott 

My bikes are set up to allow the big-big combo, as it's occasionally nice to go one gear lower without shifting to the small ring. That nicely pre-empts any concern about "what could happen" if I "accidentally" shifted to it.

That leaves the small-small, which is merely noisy if I get there by accident.

Phamilton

A standard chain has how many links? I don't remember. My Voyageur takes the whole chain exactly, no links added or taken away. Is that what you mean by set up to allow big/big, by chain length? Or something else?

ThermionicScott 

Yeah, the chain is long enough so the RD doesn't pull tight in the big-big combo (it probably has one full link of surplus.) If I hadn't cut the chain it would be droopy for at least half of the cassette when in the small ring. I also chose as short a bottom bracket spindle as possible so that the big ring would line up close to the center of the cassette. An alternate way of thinking about my gearing system is as a 1x with a granny ring.

When I built up a mixte for my wife, I used a long-cage RD and threw a chain on without cutting it, and it works perfectly, even though it's longer than necessary for the gear range.