Some of us give full rights. I typically prefer a sure sell of custom illustration work than putting obstacles and due to this get then fewer gigs. Luckily, I'm busy, as I attend many niches/fields, just hit very recently self published writing, but I don't want to revive that feeling of the beginning when not having work for several days. Sometimes an entire week... I had to research myself about it, right now I'm a bit more versed in legal stuff (but not a lot), because some people willing to get a custom illustration (or game art, logo design, brochure, website, or etc. But each field has its peculiarities in agreements, of course) are aware of this, already bring a contract, and then you review it, add whatever you need there (also as a proposal) and often request some details removed or modified. For me, is usually practical just to get them a template, as this has been studied by many, obviously, and there are very good templates covering a lot, I choose very simple ones (one or two paragraphs page), I often remove too restrictive things, and tend to go perfecting the papers. Is rare to deny something requested by them, unless being quite unreasonable. Some people think this is a hassle, but is not. You can sign the doc in the end digitally through Acrobat, or some other, or even the old simple print , sign and scan, then send it (It has legal value. Even more, probably. I'm not a lawyer...).
Very often people don't want to deal with a contract, NDA or whatever, and that's fine...
The thing is : Each artist has own red lines... Mines are: I want to be able to show the image as portfolio (in my site or social media), at least a non printable (or in lower resolution than the final file), usually a reduced JPG sample to show as portfolio. I believe most laws in every country do protect this right, just like the one of always being your authorship . Mostly meaning that the writer can't (or shouldn't) claim is the creator of the illustration. Which isn't something needed at all by the author, anyway. And the other only second thing I tend to request in the contract or whatever the written agreement, and if there's no contract, I ask it in the mails, is that the illustration is not going to be used to be sold as part of a catalog of images (for example, for a premade covers selling service, or any image catalog stock site), but fully unlimited rights for usage for the author's book, author's marketing, social media, etc, etc, and for ever. The two rights I mentioned are merely obvious (I believe the first is "automatic" by US Copyright laws), and the least thing one could ask... But stuff happens....So, when possible, I like to mention it. Both in a contract and in some email. Is not something that keeps me awake at night, though ! If something gets blatantly copied, I might take actions, or might not (probably is the author who will do so towards whatever other writer). Is none of my main worries, although it hurts if you see your custom work used in a catalog, as par of a texture pack or sth, without my permission and the client's, of course.
For me, it is even easier than go fine tuning on every possible domino effect on rights limited by time, type of media, size of audience or printed copies, etc. And in the other aspect of it, well... Heavily customized work, painted from scratch, not even from catalog photos, is not that reusable in anything else, or I see it like that. Also, I don't think is a good deal for any client if I make a custom illustration, and is something to be found later in a number of publications, or as part of other pieces, etc.
But I absolutely see the point on setting prices according to rights, licenses, type of usage. I can see how that's the way to really make a business out of it. I just believe there's room for different takes at the whole matter. Each one's circumstances weigh in, strongly. As also do own motivations, preferences, etc.