Some time ago there was an SQL-injection exploit in Drupal. The issue was tracked to a combination of issues including PDO. The problem is typical for PHP/MySQL developers attitude towards security, which seems to be "f*ck it".
One reason why prepared statements exist is that they split the parsing of the query syntax from the data. Parsing them separately means they cannot influence eachother and nothing you can put into the data will ever change the way the query is parsed.
This all must have seemed quite silly to the creators of PDO, who decided to implement their own version of prepared statements, which did not suffer from the roundtrip delay or the one-query-at-a-time limit. Unfortunately that does mean that the values are not validated, and that you can still inject anything you want into the query.
The article I refer to mentions that everybody makes mistakes, which is true, but what the PDO guys did is not a mistake, they went out of their way to make a feature that did not implement any of the safety features that people use prepared statements for. And not because they thought that they were not safe enough, but because they thought it was too slow and inconvenient.
That's stupidity of a whole other level.