Here are a few things I can think of:
1. My Dropbox folder of paper pdfs is a total mess, only made manageable because I use JabRef to open them. From within LiquidText, I have a hard time finding the papers I want. Being able to find papers again at all seems to be the main productivity boost in my math research right now, and JabRef has been great for that, even though it doesn't do fulltext search of the pdfs themselves. One important feature, here, is that JabRef helps me see papers in some historical context -- both when they were published and which other papers I was looking at when it was added to my bib file.
2. I consume lots of papers (but not necessarily in careful detail, making LT features potentially less useful), and I found the friction of having to import and manage the library within LiquidText to be a little too much for me, especially through a touch interface.
3. I like having multiple papers in front of me simultaneously. On my desktop, I tend to have multiple documents open across two monitors. I also like being able to use a physical keyboard and to have the precision of a mouse. It's nice that alt-tab (or command-backtick) is a nearly instantaneous operation.
4. Since I don't use my iPad, it tends not to be charged, leading to less use. It's also a model that can't use an Apple Pencil, so I haven't been incentivized to move all my note-taking and thinking to tablet computers.
I would really like a pdf document viewer like LiquidText for both MacOS and Linux. I put my e-mail address in for hearing about the MacOS version.
I also think that I should be using it more. I might just be lazy and not using the right tools.
By the way, Jofish was the one who recommended LiquidText to me. I forgot to also follow up on his advice to send you some immediate thoughts I had in 2016; I'm not sure they'd still be useful, but here they are anyway:
> LiquidText really is good, and it makes me (mostly) happy reading textbooks digitally. My only complaint is that with long books, say 500 pages, I have a hard time scrolling to an exact point because it becomes too sensitive, but I'm not really sure how that could be fixed (maybe a reader device which a hollowed out book you can open up to a particular page? or maybe pointier fingers?) Search-and-pinch is one of the main ways I skip around, though creating temporary comments works well, too. I find "finger-between-pages" double-scroll causes it to crash too often with Springer pdfs, so I've just avoided that feature. Maybe what characterizes LiquidText is that it doesn't disincentivize jumping back and forth between different parts of a book too much.
> Every once in a while, if I want to think of buying something, I click the "multi-doc" button to see the advertisement. Though eventually I'll probably give the LiquidText people money just to support them.
(I did end up paying for some extra features.)