If you are sitting next to someone on public transportation who irritates you...

[Alex]

If I'm remembering correctly, only in sacred writing (such as the Quran). My recollection of the history is that they were originally part of the writing system (and thus the Quran) but that over time they were dropped and only written if there was ambiguity, but when printing was introduced to the written word, the complexity of it lead to dropping them completely except in sacred texts where alteration is forbidden. Arabic keyboards generally don't even have the ability to produce short vowels. The complexity of Arabic printing comes from the fact that the exact shape of a letter changes based on the letters than come before and after it (as well as the position on the horizontal line). For example this bit consists of four letters:

لداب

and this is the same text except the second letter has been removed resulting in a completely different visual presentation of the first and third letters (remember, read from right to left):

لاب


But at least they have word breaks.