We've been getting books from Imagination Library for a couple years have really enjoyed the service. It's easy to get into the routine of reading the same books over and over to kids. This at least gives us some variety and is obviously much cheaper than something like Amazon's Prime Book Box.
I do feel guilty at times because we aren't in financial need of the books. However, I do plan on donating back to Imagination Library, which will probably more than cover the cost of the books they've sent us over the years. Maybe that's part of their reasoning for not making the program dependent on financial need.
Don't feel guilty. The other thing you can do is donate those books forward (assuming they've not completely fallen apart), to a community library, or a local school. Children's books, especially for younger kids, can take a real battering, so new supply is often welcome.
My son's primary school has a really extensive collection of books you can borrow, all of them donated by past parents. They have them on displays outside the appropriate classrooms and you just take whichever ones you want and bring them back when you are done with them. No need to check them out.
One of the reasons projects like this don't always means-test is because it puts people off asking. A lot of people see it as shameful, it can be invasive and more time-consumeing, having to say "I'm poor, here's the paperwork and supporting documents to prove it, please give me help". In addition, the savings from limiting it are often partially or totally outweighed by the additional cost of all that beauracracy.
In the UK there is an additional payment that goes to all pensioners in winter to help with the cost of heating. It's automatic because there was a real likelihood that making it means tested would simply mean that many who could benefit would never apply and the admin would cost more than was saved. I think it was the same argument for making school meals free for all kids for the first few years.
I do feel guilty at times because we aren't in financial need of the books. However, I do plan on donating back to Imagination Library, which will probably more than cover the cost of the books they've sent us over the years. Maybe that's part of their reasoning for not making the program dependent on financial need.