To Be Completely Fair

to both St. Thomas Aquinas and the scholastics (contra another comment at Disputations) I quote:

One of the favorite things to ridicule is the supposed debate among the Scholastics on the question of "How many angels can dance on the head of a pin?". Apparently, however, none of them put things in exactly these terms, as those concerned to rescue the reputations of Aquinas and the others are anxious to emphasize. The Scholastics could have very reasonably focused on this funny question, however, for it does concentrate several of their points of dispute, including whether "angels" have a corporeal (bodily) or merely spiritual existence.

And in fact, some of the Scholastics, such as Aquinas, did dance quite close to the precise question, as this little taste from his "Summa Theologiae" shows:

Q. 52, a. 3 - "Whether Several Angels Can Be At The Same Time In the Same Place? There are not two angels in the same place. The reason for this is because it is impossible for two complete causes to be immediately the causes of one and the same thing. This is evident in every class of causes. For there is one proximate form of one thing, and there is one proximate mover, although there may be several remote movers. Nor can it be objected that several individuals may row a boat, since no one of them is a perfect mover, because no one man's strength is sufficient for moving the boat; the fact is rather that all together are as one mover, in so far as their united powers all combine in producing the one movement. Hence, since the angel is said to be in one place by the fact that his power touches the place immediately by way of a perfect container, as was said (Q. 52, a. 1) there can be but one angel in one place."

The original source in its entirety.

That said, I will point out that even the point made here by Aquinas has vanishingly little relevance to how we are to conduct ourselves as Christians, and that is the point of the mockery "How many angels can dance on the head of a pin?" In a sense that study becomes so abstruse that it becomes disconnected from the reality of living and hence, useless.

That said, the questions about angels comprise a minute portion of the Oeuvre produced by St. Thomas Aquinas. While some of the other questions may have similar small relevance, there can be no denial of the immediate importance of the vast majority of his work. There are probably many "hobbies" of Saints to which we could take exception were we so inclined. I don't see how speculations about angels are out of order in the enormity of the serious and focused work done.