Thatcher’s Arabic Grammar has largely been made redundant by the improvements and additions made to it by Haywood and Nahmad.
Thatcher may still be of some use to anyone who wants additional translation practice, particularly if you are keen to acquire a more classical vocabulary, as one of the alterations made by Haywood and Nahmad when they updated Thatcher’s Grammar was the inclusion of more modern vocabulary, so that the book could be used by both students interested in classical and those interested in modern Arabic.
Apart from that, the book is really of historical interest only, as a major work of Western scholarship in Arabic.
In short, if you have the money to spare, by all means get it – it’s nice to have read through it, and compare it to Haywood and Nahmad, and, as mentioned, it does have a slightly different vocabulary list and therefore different translation exercises, which may be useful. Otherwise, go for the latter book.
One thing to note in particular is that the edition published by Hippocrene Language Studies does not have a particularly good Arabic font, which will slow your progress unless you are already accustomed to reading Arabic, and have a reasonably good vocabulary already memorised. Unfortunately, we have not been able to peruse other editions for comparison.