(1) In 5E, wizards are less spellbook-dependent than they used to be (compared to AD&D 2nd edition, anyway--I haven't played 3E or 4E) and spellbooks are relatively cheap to create. Making a backup spellbook doesn't take you six months of your life and thousands of gold pieces; nor do you absolutely need to have a spellbook all the time. In fact, you can feasibly even leave your spellbook at home for a whole weeklong adventure against orcs or something and manage just fine--you still get all your spell points/slots back every time you long rest.
(2) Drawmij's Instant Summons doesn't help that much if the spellbook does get stolen. "Bohab the Tishabite has your spellbook; he is in Londonium" doesn't help all that much. It's barely enough information to get you to the lowest level of scrying ("secondhand: have heard of the target"), which means the target will have +5 on its Wisdom saves, which basically means that you can probably scry out a mook after a couple of days of trying, but if the spellbook has already been passed to someone with a strong mind (high Wisdom save) you may never be able to scry them out.
Excellent points. I know you said you're convinced, but if it's all the same I'm gonna continue ranting anyway. I would still think it's worth it if you happen to have DIS as a spell (I'm not sure I would ever spend my spell selection on it).
Let's assume that you are a level 11 wizard (because it's the lowest level at which you can cast DIS), and that your spellbook contains only the spells you have gained from leveling up plus DIS, but none of those spells are of your school (or you are playing an arcane tradition not found in the PHB) and each spell is of the highest level you can cast when you learn it. That's 82 levels of spells, which is worth 870gp + 82 hours of scribing (which probably comes out to a week and a half worth of time, because it's probably not consecutive). That's almost the cost of the sapphire already, plus a large chunk of time that could be spent investigating post-scry assuming it has changed hands. If you have added at least 13 more spell levels worth of spells, you officially break even. Even then, with DIS you still have the chance of removing the time cost if you happen to crush the gem when it is unheld, I imagine your chances are better at night. If I have to spend about a week and a half doing something to ensure I continue to have my spells, hunting down the thief who took my spellbook sounds like a funner adventure than spending the same amount of time writing a new one. Another thing to consider: they stole your spellbook, but was that all they stole? If your book is held when you crush them gem, and gives you a target to scry on, that's a lead to the rest of your stuff too. Furthermore, stashing a spare spellbook away means you need to retrieve it every time you gain new spells if you want it to be up to date. Getting back to our example wizard, given the conditions described above and assuming Int 20, you have 16 spells prepared (and of course you have your trusty cantrips) while your book contains 27 spells, so if you lose your spellbook you're by no means crippled, but you just lost 11 spells (probably all your rituals) which is still pretty disappointing. Wizard's power is in the versatility of their huge spell list, so leaving your book at home means you can't take a "quick" 8 hour break to swap out a spell if something comes up, as well as losing your instant access to rituals you have scribed.
tldr; I would use it if you get handed more than a few spell scrolls and spellbooks from other wizards. Of course you could scribe another book just to be double sure.
Btw, since we've had similar back and forths a couple times this week or so and I don't post often I just wanna make sure, I'm not being too annoying or coming across as aggressive or anything like that, am I?
Side note: I totally forgot both that copying from your own book is considerably cheaper and that DIS doesn't bring you things that are held until I double checked after reading this topic. I'm learning so much!