Many forum systems use external libraries such as HTMLPurifier, Zend and so on. While they are great, I do feel that that they come at a cost: you are relying on the 3rd party developer to update their library to support new standards and in the case of the two named libraries, new versions of PHP. Last time I checked IPS 4.1 wouldn't work on PHP 7 because of HTMLPurifier, and IMO that would have been a bad thing for IPS. Had IPS be using their own custom written library/function, this wouldn't had happen and if it did, IPS would be able to fix it themselves. What do you think about the cost of using external libraries and do you think that it worth the risk?