Motivation is the long-and-growing list of bugs/inconsistencies/eccentricities in PHP's DOM implementation. See T215000: Fill gaps in PHP DOM's functionality, T217766: Flow\Exception\WikitextException: ParseEntityRef: no name, the existence of MCS' HtmlFormatter library (T217360), etc. for details.
An alternative is to port domino/etc to C directly and have it be usable as a PHP extension so we get good perf as well. If it used libxml's nodes underneath you could still do fast XPath queries, etc, using the existing DOMXPath package. (On the other hand, the relatively fast pace of change in the DOM WG may mean that tying this to the PHP release cycle is not the best idea.)
A note about priority and dependencies: We are not going to do this as part of the Parsoid port. At this time we believe that we understand Parsoid's usage of the DOM well enough that we can workaround the bugs in the core PHP DOM implementation. But as a longer-term goal this would enhance maintainability and allow us to remove workarounds.