WatchTech

I've written about this extensively in the past as have others more knowledgeable than myself, but perhaps it's not so easy to find the posts and/or it's just an interesting topic to chat about from time to time.

Traditional, lateral clutch chronographs are thought to suffer undue wear when left running for extended periods primarily because of the funny shape of the teeth of the chronograph wheel.  These teeth are very pointy so that the clutch will engage as reliably and consistently as possible, but pointy teeth are not ideal for power transmission, so perhaps suffer more wear than teeth that are designed for continual engagement.  I can't say I've seen instances of these pointy teeth being badly worn from being left running all the time (more often the tips get squashed by careless watchmakers), and as others have mentioned, the chronograph wheel is not driving anything else other than the minutes counter, so there's not a lot of load on it outside of the drag of the friction spring (to keep it running smoothly rather than skipping and stuttering).

Vertical clutch chronographs are conversely believed to NOT suffer undue wear when left running because they have normal shaped teeth, etc.  Many of them do have a friction spring to keep the hand running smoothly however, so there is SOME extra wear occurring when those are left running all the time as well.  

Some of them (Piguet 1185 and variants (Including the Omega 33xx versions) and Seiko 613x) have the chronograph clutch smack dab in the middle of the power train, so not only do they not need a friction spring, but there is a steel wheel riding on a steel arbor when they are not running.  This makes for a defensible argument  that there is actually less wear overall when the chronograph is running. At least in the gear train proper; the minutes counter and hours counter will obviously suffer more wear when the chrono is running.  Again, those components are under very little load, so wear should be minimal.  

The problem that was hinted at about the second hand slipping has to do with the clutch spring inside of the vertical clutch getting weak and allowing the clutch plates to slip against each other when the chronograph is running.  Owners of Seiko 613x watches will sometimes observe that the chronograph second hand will get out of sync with the continuous seconds hand on a reference timepiece even though the minute hand appears to be keeping good time.  Essentially at every lock of the escapement, the clutch plates will slip a tiny amount and the chronograph seconds hand will lurch forward a tiny amount more than one second worth of travel, causing it to not keep accurate time.  It will not cause a problem with how the second hand resets however.  This "speeding chrono seconds hand" problem could be caused at times by some oil getting into the clutch (or even leftover cleaning solution) or perhaps because the clutch spring has gone weak from fatigue.  Ironically, spring fatigue of this sort could conceivably be exacerbated by leaving the chronograph OFF all the time, which keeps the spring in a compressed state.

I generally do not recommend leaving any chronographs running all the time.  It's not what they're for.  That said, it's better to do so with a vertical clutch chronograph than with a lateral clutch chronograph.

_john

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