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I don't see any problem with using Calibre as a backend as long as they abide by the terms of its license (GPLv3?) which if they're just executing it, is fine.
Makes a lot of sense to not constantly re-invent the wheel. Reuse what you can, improve on the rest. Would be nice if the result was an open source project too though. All things being equal (sadly they often aren't), I'd rather use an open source product than closed.
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