I was wondering why some Esperantists use "x" system(it is not in Esperanto Alphabet) instead of "h" (that is in Esperanto alphabet). It shouldn't be promoted "h", regularized or something like that?
Two arguments I have encountered for preferring "x" to "h":
Because it is not an existing letter in the Esperanto alphabet, automated conversion is possible, e.g. sx could be unambiguously converted to ŝ. By contrast, even if combinations like -sh- are rare, they could occur in compound words e.g. dishaki, so sh cannot be reliably converted to ŝ (similarly senchava, flughaveno, etc).
Because it is so near the end of the alphabet, it is almost always the case that a naive lexical sorting will achieve correct alphabetical order. So for example, a naive sorting would put sxi after sorto (which would be correct, because ŝi comes after sorto), but would put shi before sorto. This only fails when a letter to which an accent could be applied is followed by z; for example a naive sort would misorder uxato (ŭato) and uzi, but such occurrences are very rare.
People tend to prefer the x-system exactly because it isn’t a letter in the Esperanto alphabet, so there is no risk of confusion. For example if you see the word flughaveno you might first ask yourself ‘what is a fluĝo?’ before recognising the word. With the X-system there is no such ambiguity.
These days the hats are pretty widely supported in browsers and most software, and it’s usually not too difficult to install some software to help type them, so both systems are less and less common.
I personally prefer the more natural h (ch, sh) and such a notation was proposed as alternative for Esperanto letters.
In the newer times the alternative with x was introduced with as argument: automatic translation. As
chashundo was ambiguous
chas-hundo. Try reta-vortaro.de with x's.
It is a subcultural phenomenon I think. Like accented characters are very dear to the language users. German without umlauts, a French café without accent droite, Spanish ñ. Even English borrowing façade.
Both are legal and understood. So use h with hyphens (chas-hundo) if it pleases. Meanwhile I am content to use x too.
By the way for smartphones one can install Esperanto support for the letters.