Worth (compare Like) has a prepositional force, and needs an object. This object may be either while (i.e. the spending of time) or something else. It is therefore correct to say: "this job is worth while"; it is also correct to say "this job is worth doing". But one object is enough, and so it is wrong to say: "this job is worth while doing".
Worth-while as an adjective ("a worth-while job") has not yet reached more than colloquial status.
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