This feature has been DEPRECATED as of PHP 5.3.0. Relying on this feature is highly discouraged.
PortabilityAssuming it to be on, or off, affects portability. Use get_magic_quotes_gpc() to check for this, and code accordingly.
PerformanceBecause not every piece of escaped data is inserted into a database, there is a performance loss for escaping all this data. Simply calling on the escaping functions (like addslashes()) at runtime is more efficient.Although php.ini-development enables these directives by default, php.ini-production disables it. This recommendation is mainly due to performance reasons.
InconvenienceBecause not all data needs escaping, it's often annoying to see escaped data where it shouldn't be. For example, emailing from a form, and seeing a bunch of \' within the email. To fix, this may require excessive use of stripslashes().