One should be able to choose which editor to use and should not be confined to the default built-in editor.
My initial reaction is, "well...actually, no," because Xiphos' editor is necessarily aware of internals that no outside editor would reasonably be expected to provide.
For examples, Xiphos' editor supports linkages using Sword-internal (in fact, Xiphos-internal) mechanisms. There are 2 types of such linkages, one which kicks a set of verses out to the verse list machinery, while the other induces immediate navigation to one passage, Bible or not. I have some trouble envisioning any other editor knowing how to construct those. There is an enhancement planned to provide footnote creation as well, which again employs markup specific to our internal format.
In general, we can guarantee that a document provided via Xiphos' editor is proper, for properties unknown to any other editor. Yes, we could assume other editors might produce valid HTML (which is nearly enough in rough display terms for the markup Xiphos can show) but details like this sort of linkage are not producible by any app not sufficiently in bed with Sword.
I'm open to reconsideration if a good enough (general enough) use case can be offered, but this is the first time anyone has ever suggested employing an outside editor.
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no followup; can re-open (or re-file) if someone provides a sufficient use case to justify the effort.