Obryn
Hero
First off, if there's THAT many monsters with spells, that it would cost $20-$30 more to print a book with some capsule summaries, then ... wow. That would mean two things: (1) you're darn right I want those capsule descriptions right there if that's what you're going to be using for that many monsters, and (2) they should really seriously cut down on the number of spells per monster, because that's a lot of avoidable overhead and complexity. Oh, and I guess three things - (3) holy cow am I not running 5e without capsule summaries if there's that many monsters with spells!As previously noted they've added 32 pages to the MM just to include all the monsters they've designed. By not raising the price WotC is already reducing their profit margin.
If you then add spell descriptions you're expanding the page count even more! Understand that the cost of paper is the single biggest expense in production.
I agree that it would be more convenient to have a short description listed with each spell in each monsters stat block. I'm absolutely sure I'd not be willing to pay 20 or 30 dollars more for a book that's already 50 dollars for such a minor convenience.

Second, assuming there's some semblance of a reasonable amount of monsters with actual spells, I'd be perfectly happy either paying $5 extra or taking 10% fewer monsters. At-table usability and reference minimization are seriously that important to me.