If people (monks) spent all their time writing and drawing funny pictures in the margins, why are they not that great at drawing?
At least judging by the way this question is phrased, it sounds like it's just a matter of taste, which we can't really help you with. I happen to adore the illustrations in illuminated manuscripts. The Winchester and Morgan Bibles are two of my favorites.
In the medieval mindset, the demarcation between the sacred and profane spaces was very important. The heavy stone walls of a Romanesque church provided a physical boundary between the ordinary outer space, and the sacred interior, where the relics of the saints and martyrs created a spiritual connection between the faithful and the heavenly world. Colorful frescoes and mosaics on the walls, and bejeweled reliquaries and crosses on the altars, identified this inner church space as very different from the world outside those walls. The church was a model of the Heavenly Jerusalem, which the biblical vision of Ezekiel described as a world of light and colorful gemstones. Those medieval churches identified the sacred through their materiality, and even more so the gothic churches of the thirteenth century and beyond, with the amazing display of colorful stained glass on their tall windows, and the apocalyptic scenes depicted on the tympanums over the church entrances.
Religious books were another medieval sacred space. The words of the bible and of the early fathers of the church were used in liturgy and written on vellum, literally made of flesh, and representing by its very materiality the incarnation of the Christ. In the early middle ages, both in the Byzantine east and in the Roman west there were long and detailed theological discussions about the relative relevance of the sacred words and of the images that attempted to portray those words. In Constantinople, this led to the iconoclasm of the eighth and ninth century and the destruction of a large part of the religious art within the borders of the eastern roman empire. But even in the Carolingian west, the written sacred text was always seen as holy, while the religious narratives in churches and manuscript illumination were seen as less significant, and only good to represent the saints and to show the physical nature of the Christ that had walked the earth.
Religious manuscripts of the early middle ages were not meant for individual reading and contemplation. They were often large books, most often used as aide-memoire for those who were reading aloud the words of the psalms or the gospels to the congregation or to the fellow Benedictine monks. In spite of the primacy of the written word in the medieval church, that was still a very oral world, where individual, silent reading of a book was almost unheard of, and text was memorized and chanted aloud. The book itself was a sacred space, often encased in a cover of gold and gemstones just like the precious reliquaries would encase the sacred bones of a martyr.
In the eleventh century, both religious and lay society starts to change. Over the centuries of the central middle ages we see a significant growth in literacy and the birth of a merchant class that could afford to commission illuminated manuscript. Lay religious passion manifests itself in many ways in this period, from the growth of pilgrimage to the holy sites, to the rise of the mendicant orders, the Franciscans and Dominicans. Individual reading of the bible, without the intercession of the clergy, becomes common, albeit resisted by the church. Devotional books, psalters and book of hours especially, were central in that individual search of a connection with the holy through the written word.
As we see more and more religious texts in the hands of the newly literate, wealthy merchant class, marginalia begins to appear on the pages. The odd images of snails and rabbits jousting, grotesque pictures of deformed, bearded men and even nuns collecting penises from trees, look very alien to us today. Surely it must have been a rather random waste of precious space, although often quite funny. But they were not really random. They appear in prestigious books which were already quite expensive to make and illuminate. Any additional illumination, including an odd, seemingly unrelated image on the margin of the page, would add to the cost.
Their purpose was indeed similar to that of the walls of the church, to demarcate visually the boundary between the sacred space of the written words of the bible, and the outer, profane space of the physical human existence. Sometime they would provide a visual gloss, a commentary on the words on the page, but more often than not they were clearly distinct from the religious holy text, and directing the faithful to focus on the holy space on the page, to find there the spiritual connections that they were looking for.
For more on medieval art and the essential function of light, color, and the demarcation between sacred and profane, I highly recommend:
Kessler, Herbert. Seeing Medieval Art, Toronto: University of Toronto Press (2011)
