For one thing, the descriptive power of the base component does not readily tolerate irrelevant intervening contexts in selectional rules. If the position of the trace in (99c) were only relatively inaccessible to movement, the speaker-hearer's linguistic intuition is, apparently, determined by a descriptive fact. Suppose, for instance, that the systematic use of complex symbols cannot be arbitrary in problems of phonemic and morphological analysis. Comparing these examples with their parasitic gap counterparts in (96) and (97), we see that any associated supporting element delimits a general convention regarding the forms of the grammar. Notice, incidentally, that relational information suffices to account for the levels of acceptability from fairly high (eg (99a)) to virtual gibberish (eg (98d)).
see also: WikiPedia -- Chomskybot