A contract with the wording "work for hire" does, indeed, mean that the client gets everything -- rights, originals, etc. You are basically a 'hired pen'. If it doesn't matter to you whether or not you can ever use the illustrations again (if, for example, the illustrations are so specific to the job that they would never fit another context), and if they are also granting you the right to use the illustrations in your portfolio (as they appear to be willing to do), then if the money is right, signing such a contract would not necessarily be a terrible thing. The operative phrase, however, is 'if the money is right'. If it's an all rights buyout or work for hire, my default is to at least double the price I would have asked for otherwise.
The company's statement that an agreement that would allow you to license the illustrations to other entities "doesn't hold for independent contract work," is not true, actually. They may be sure that THEY don't want to pay you to develop materials that you can license to others, but it can certainly be done -- I do it all the time. A contract is whatever arrangement two parties agree to.
Since the people at this company are clear that they want a 'work for hire' agreement, I would say make sure the price is worth it. They are paying by the hour, so at least you won't get stuck with a set price if they start making a ton of revisions.
Be aware that once they own all the rights, it's basically as if they created the work. Among other things, that means that if one day you were to create a derivative work from one of those illustrations, THEY could sue YOU for infringement. It's something to keep in mind as you work: if you had to create another illustration of the same subject, how would you do it differently? You might even want to keep some of your most brilliant ideas in reserve, if you know you plan to publish later on the same topic.
Good luck, and don't let 'em push you around!
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