indeed, many keep leathers w/ sps/lps corals w/no ill effects.
one or two leathers in a large enough system usually presents no problems. it doesn't mean that addressing terpene production by leathers shouldn't be dismissed, or dealt w/ pro-actively-i've dealt with plenty of hobbyists who've had issues w/ a stony coral that was in close proximity to a leather's 'upflow', only to have those issues disappear by moving either the leather or the stony coral.
MOST tanks that are successful keeping both leathers and hard corals in 'large' amounts of each, long term, usually have carbon and skimming running in their system, and won't be able to keep both groups 'happy' without it.
once you're dealing w/ a hundred(s) (of) leathers upstream from a hundred(s) of sps's in a coral 'run', the issue becomes quite clear, and irrefutable. the 'nuking' we observed took only a matter of hours (less than 24) for lots of sps 'frags' (aquacultured/maricultured imports that had already 'settled in to the system) to burn beyond repair. as soon as we moved the leathers into their appropriate trough, things settled down again. for this reason, we ALWAYS tried to keep the sarcos and sinularias in the last trough in the system before the sump/skimmer processed the water for return to the system.
a hobbyists's tank w/ one or two leathers isn't really an example that can point to anything conclusive, as far as terpene production by leathers, or what harm they can do.
there's certainly effects possibly occuring that we don't observe-just because a coral doesn't die, or *appear* to be stressed, doesn't mean it's not getting stressed
running carbon makes a whole bunch of sense, for more reasons than leather terpenes-there's *alot* of allelopathic chemicals being produced all the time by most, if not all corals and algaes-running carbon simply helps to removes the *potential* issues that can be caused by those substances. yellowing compounds are another. it's that whole 'an ounce of prevention is worth a pound of cure' thing.
am i saying that running carbon is a necessity for success? or that one's sps's are guaranteed to stress or die if you keep leathers with them in an aquarium?of course not. there are also other means/tacks one can take-like water changes, for example
i do hope that folks won't take a post saying that someone keeping a leather or three w/ sps's for 5 years is justification for dismissing terpenes, or carbon use, as a non issue
having said all that-i've also run plenty of tanks w/ either no carbon, or intermittent use only, w/ no issues-with and w/out leathers present in the system. i also performed regular large wc's and skimming as part and parcel of my maintenance regimen
hth