Another reason against it: security. You could be lulled in a feeling of false security if you have magic_quotes=On on a test server and Off on production server.
And another: readability of the code. If you want to be portable you need to resort to some weird solution, outlines on these pages (if (get_magic_quotes())...).
Let's hope magic_quotes soon goes to history together with safe_mode and similar "kind-of-security" (but in reality just a nuisance) inventions.