This comes up a lot for me, because my contract is specific about intellectual property - it goes like this:
"Subject to and expressly subject to full payment of all fees, costs and expenses due, Designer grants to Client the perpetual and worldwide right and license to use, reproduce, adapt, modify and display the Final Art solely in connection with the Project as defined in the SOW and in accordance with the terms and conditions of this Agreement. With respect to such usage, unless otherwise provided in the SOW, Client shall have nonexclusive rights."
So what does that mean for my clients? And why don't I simply grant them ownership of whatever I make during our time working together?
The standard metaphor is buying a cake vs buying the recipe… A small bakeshop selling a cake is straightforward, someone offering to buy their recipe isn’t. It implies limitation (the bakeshop can’t make that cake anymore, nor can anyone else), certifiable originality (Where did it come from? Does anyone else make a similar cake? Is it possible that two identical recipes were contrived independently of each other?) among other problems. What I offer is more like baking lessons - we figure out a recipe you like, we bake you a cake together, and we give you the means / wherewithal to make that cake or any other version of it you like as long as you like.
The simplest explanation is: clients hire designers for a service rather than buying a product or idea outright from them.
I give them license to use the ideas & artwork that come out of it, however they want for as long as they want, so long as it's connected to the project we worked on together. It’s difficult to even claim my ideas are something that can be owned outright in the marketplace... the level of 'ownership' is a function of how broad the definition is. I've had one client try to copyright / prevent me from using "artwork" on a wine label, another prevent me from ever using the same label material on another wine project. You can imagine how this would be a problem... a few projects and I'd have no options left for future clients.
I don't infringe anyone else's copyrights, I don't copy / borrow / steal, but we all stand on the shoulders of giants. My work doesn't come out of a vacuum... I don't assign rights to the client and am specific about non-exclusivity because there's so much overlap in the corner of the industry and even smaller market segment we work in. To do so would be an inaccurate representation of what I'm providing to the client, as well as a liability.
In regards to strategy, there's no unique or original offering, I'm just choosing an appropriate set of recommendations from the suite of available options. In terms of design, there are always processes, materials, design choices and other elements that will all be in common with other clients (past and future) in such a way that I can't grant rights without implying future limitation or past conflict.
Hopefully that helps clarify - Christopher Simmons has also written eloquently on the subject here and the AIGA has significant resources explaining industry best practices (our contract is based on their industry boilerplate). I'm also always available at email@example.com for further questions.