It's said that most influential medieval work on angelic order was written by Pseudo-Dionysius the Areopagite. How much authority do his works still hold and what's the metaphysical foundation and necessity for the existence of Angels?
St. Thomas's angelology based on Holy Scriptures
Fr. Garrigou-Lagrange, O.P., discussing St. Thomas Aquinas's sources in Reality: A Synthesis of Thomistic Thought ch. 22, writes:
It is sometimes thought that the treatise of St. Thomas on the angels is an a priori construction, having as its sole foundation the book of Pseudo-Dionysius, called De coelesti hierarchia. This is a misconception. Scripture itself is the foundation on which St. Thomas rests.
Pohle, God the Author of Nature and the Supernatural: A Dogmatic Treatise, pt. 2, ch. 3, §1, Article 1 "Existence & Nature of the Angels":
The existence of Angels is a truth so obviously founded in Scripture, Tradition, and the teaching of the Church that it seems superfluous to undertake a formal demonstration of it. We therefore merely indicate some of the many Scriptural texts in which it is expressly taught: Ps. 90:11; 102:20; 148:2; Matth. 4:11; 18:10; 22:30; 25:31; John 1:51; Heb. 1:4.
Proofs he's not a "pseudo"
Dionysius the Areopagite and a woman named Damaris and others with them.
Mgr. Darboy has so much data, both intrinsic and extrinsic evidence, in support of his position that the author whom for centuries councils (even heretics!), doctors of the Church, etc., considered authentic is not a "pseudo".
Opponents of his being St. Dionynius base their arguments on one data-point:
According to Stiglmayr's Catholic Encyclopedia entry Dionysius the Pseudo-Areopagite, Stiglmayr & Koch (end of 19th cen.), after Mgr. Darboy (1845), allegedly discovered some Proclus verbatim in On the Divine Names! Well, how do they know Proclus (6th cen.) didn't plagiarize St. Dionysius?