I know and I tend to agree, but the rules in Covenant for build points are stacked that way. It is a much more efficient use of build points to purchase a high-level/high-quality summa than any other kind of book.
For example, a L16Q15 summa can provide up to 136 xp in that Art (about 9 seasons of efficient study), for those 31 build points. By comparison, two L8Q8 summae will only provide you with 72 xp in the same time for the same cost in build points. Tractatus are even worse, with 3 Q10 tractatus providing only 30 xp.
So unless the storyguide or the troupe put limits (or purposefully decides to limit) the high-level/high-quality summae when designing the covenant's starting library, there is little incentive in the rules to purchse anything but those.
Now, if we wanted to go that way (and I'd be on board if we did), then a more balanced starting library could look something like this:
Sample Library #2 (274 points)
- Terram summa L23 Q12 (free from the Exceptional Book boon, on the Art that got the most votes)
- Creo summa L16 Q15 (31 points)
- Animal summa L13 Q16 (29 points)
- Aquam summa L13 Q12 (25 points)
- Intellego summa L12 Q10 (22 points)
- Vim summa L12 Q9 (21 points)
- Corpus summa L10 Q15 (25 points)
- Imaginem summa L5 Q15 (20 points)
- Mentem tractatus Q12 (12 points)
- Magic Theory summa L6 Q15 (33 points)
- Finesse summa L3 Q15 (24 points)
- Penetration tractatus Q11 (11 points)
- Code of Hermes tractatus Q10 (10 points)
- Magic Lore tractatus Q11 (11 points)
By Covenant's definition, that would give us 3 "sound" summae (Creo, Animal and Magic Theory), 6 "vain" summae and 4 "vain" tractatus. For exactly the same cost as the first library. It still has many holes, but a few more subjects are covered.