I sometimes use regular scriptures in hard copy, even though I have a couple tablet. Here are my reasons:
1. I'm light-sensitive. The less artificial light I'm exposed to, normally, the more I feel the Spirit in church (and the less ill I feel). Tablets and smartphones produce a lot of artificial light. Also, the flickering that happens when you scroll up and down a chapter or such is something I'm somewhat sensitive to (that's not the programmer's fault; it's just the nature of scrolling). I'm actually *more* light-sensitive to the lights that light the room in church, too (but the combination of that and glowing screens within arm's distance is harder to deal with than one or the other).
2. It's a lot faster for me to look up scriptures in my regular scriptures sometimes, because it takes a lot of steps to navigate through the menus to get there. If I'm following along in the teacher's manual, it's a lot faster, though, since I can click the links from there and directly get there. Using the menus is not equally fast on every device. I use a couple Kindle Fires for the app.
3. Physical scriptures don't require electricity or the Internet.
Here are my proposed solutions (the numbers correspond to the issues above):
a) Make it so you can optionally flip pages instead of scrolling up and down (I don't mean by the physically printed book's page, but kind of like the page-down/page-up button on a PC, or moving by page in a Kindle e-book). This removes the flicker issues, since it only needs to refresh once per page turn.
b) Allow a custom font color scheme (instead of just a few options). I like reading with night mode (a black screen produces less light), but I would prefer a less bright font in many cases (I might like to make it red, orange, tan, dark gray or such). Custom background colors and fonts would be nice, too (but less of a priority than the aforementioned).
c) Lobby for more devices that don't require a backlight or glowing screen (the light in the room is enough for some people). As long as screens have to be glowing, some people are going to prefer a regular book. Yes, I'm aware of e-ink devices, although current popular ones don't fill the need (and won't support an Android OS or such; I have heard a rumor about an e-ink Android smartphone, but that's it). There are alternatives to e-ink, though. Anti-glare in sunlight isn't a must (there's not generally a lot of sunlight in church).
2. Make it so you can look up scriptures, talks or such, by typing them in a query box directly (instead of only by navigating through menus): e.g. typing '3 Nephi 10:16-18' will bring it right up. Menus can be pretty slow to navigate through (especially on slower devices). I like typing stuff in to make things happen. Menus and clicking and tapping all the time aren't my thing (although they're not as bad on a touchscreen).
a) Make it so the app takes less battery life and processing power, if possible (this doesn't remove the need for electricity, but it helps). I've noticed that when I use the aforementioned scrolling feature a lot, the battery drains faster. So, if you add a page-turning option, that would increase efficiency.
b) Make it so the device respects a person's wish to be offline and still use it (it told me I needed to have an Internet connection to use it once, although I don't know if that's the norm, now). It never used to prompt me to connect.
c) Make it so you can disable automatic updates. They can get someone off course when they're trying to open up the app to look up something in Sunday School, sometimes. A regular book isn't going to prompt you for updates. Updates are great, but I sometimes like to decide when and where to update.
4. Add more features of things you *can't* do with a regular book.
a) Make it so you can look up highlighted words in a dictionary on your device and/or online with en.wiktionary.org. Bringing a physical dictionary to church with you is possible, but it might be really bulky. The Bible Dictionary and such are great, but there are words that aren't in it.
b) Continue enhancing the audio scriptures. There has been some great progress, lately.
c) Make it so the app can notify you when class is over.
d) Make it so students can connect with the teacher's device for anonymous or onymous polls and such.
e) Make an app that allows users to see the teacher's screen on their own phone or tablet (so everyone can literally be on the same page without having to look up stuff), and/or so the teacher can signal everyone's connected tablets to navigate to the correct location at a whim. Yes, this is possible.
f) When you update the app, try not to remove cool features, if possible, unless you have to. Books keep the features they have (generally), and people can learn to rely on them. If you're trying to simplify the interface, you can always hide the features you would remove, and make an advanced mode for those who want to use them. If you think that will still be confusing to people who might enable it who shouldn't, you can make it pop up a warning when they try to enable it to let them know it might make things overly complicated for regular users.
g) Add a countdown timer (which would be useful for teachers and readers who want to read or have the class read something for a specific amount of time). I have a separate countdown timer app that I use.
h) Allow users to study together (two or more) via their device. So, you can compare each other's highlights and notes. These should be different highlights and notes than you use standardly (another scheme, with what we have now being private). Switching highlight/note schemes would be nice (and having a unique one for each study group). Members of the group would have their devices paired in a fashion.
Anyway, these are some ideas for you, if you like them.