On the one hand, those who promote the external aspects of divine worship would note that these are important for a variety of reasons, not the least of which that these exterior aspects do move us interiorly and also speak to the importance of what underlies these rites and ceremonies; that these things are aids to teach us and form us and have an evangelical aspect to them; that they are an inspiration and a help and, further, give glory to God.
On the other hand, those who express concern about what we might call "externalism" are expressing the concern that people not fall into any temptation of turning the sacred liturgy or their faith generally into a merely aesthetic experience devoid of any deeper and substantial root. They would emphasize that one cannot solely be concerned with beauty but must make the ultimate priority the worship of God and a life rooted in the Gospel.
In point of fact, and far from being opposed, both have something quite important to say, rooted in the recognition of some important (and complementary) truths. Both of the expressed views are rooted in a consideration of the heavenly and the divine and what both stand against is in fact the common enemy of a reductionism related to beauty. That reductionism is rooted in two different extremes coming from opposite ends of the spectrum, but the common thread that unites both of them is that both have placed barriers between the exterior and interior aspects, accordingly approaching the externals in a shallow way.
Let us consider how Dom Columba Marmion explained the matter of their relationship in his work, Christ in His Mysteries:
There are some minds who see nothing else in the celebration of the mysteries of Christ than the perfection of the ceremonies, the beauty of the singing, the splendor of the vestments, the harmony of the rituals. All that may be there; all that is encountered indeed: all that is excellent.
In the first place because, the Church having herself settled all the details of the worship of her Spouse (this Church who is the spouse of Christ), the perfect observation of those details honors God and His Son Jesus. “It is an established law for all the mysteries of Christianity that, to reach our understanding, they must first be presented to the senses: and this had to be so in order to honor Him who, being invisible by nature, willed to appear, for love of us, under a form that was perceptible.”
Further, it is a psychological law of our nature—our nature being matter and spirit—that we go from the visible to the invisible. The exterior elements of the celebration of the mysteries are to serve for our souls like the rungs of a ladder, so as to raise us to the contemplation and love of realities that are heavenly and supernatural. This, furthermore, is the plan of the Incarnation itself, as we sing at Christmas: “... so that while we recognize God in visible form, we may through this be seized with a love of things invisible.” [Footnote: The holy Council of Trent expressly teaches this about the rites of the Mass, about the primordial action of the liturgy: “As the nature of men is such that they are not easily able to meditate on divine things without the support of exterior aids, our holy Mother the Church has on that account instituted ... certain rites and brought in ceremonies by which the minds of the faithful are stirred up by these visible signs of religion and piety ... to the contemplation of higher things” (Sess. XXII, c.5). This teaching can perfectly be applied to the whole of the liturgy.]
These exterior elements, therefore, have their use, but we ought not to rest upon them exclusively; they are but the fringe of Christ’s garment. The glory, the splendor, the power of the mysteries of Jesus are chiefly interior, and are what we should be seeking above all. More than once, the Church asks God to give us, as a fruit of holy communion itself, an understanding of the power special to each mystery, in order that we may enter deeply into it and live it: “... that what we celebrate in solemn office, we may attain through the discernment of a purified mind.” This is knowledge of Christ as St. Paul wishes it: “in all wisdom and spiritual understanding.”
-- Dom Columba Marmion, OSB, Christ in His Mysteries
Here, in Dom Columba's words, we see the proper balance struck; a balance which acknowledges the relevance, pursuit and importance of the external aspects of worship, while at the same time pointing to the priority and necessity of the interior to which these things point.
Those who would like to read more from Marmion's, Christ in His Mysteries should know that this work has been reprinted by Zaccheus Press.