A-llah is the Arab short form of al I-llah (The G-d). The addition of the definite article al (the) was meant to refer to the One G-d, since I-llah was also used to name idols. Similar to the Hebrew "E-lohim acherim" in which the word is used to refer to idols.
Since Muslims use A-llah to refer to H" or at least claim to refer to Him, and the word is also used sometimes by Christian and Jews who speak Arabics, it should not be pronunced or written in vain.