It's most like a theme clue -- in an American-style crossword you often have 1 to 4 extra-long answers, almost always horizontal, that are related to a theme (they might be puns, or proverbial sayings, or together spell out a longer quote). It's sometimes possible to solve a theme clue in a crossword when you have only a few letters, if you understand the theme well enough.
In this case, the "suddenly solving the whole thing given only 40% or 50% of the letters" effect can be kind of cool, or annoying if you were enjoying the logic puzzle aspect. In the original MIT Mystery Hunt hexagonal regular expression crossword, most of the grid did not spell anything recognizable, so it was clearly a logic puzzle through-and-through. I guess whether people appreciate that depends on whether they were expecting a pure logic puzzle or a combined logic+word puzzle.
In this case, the "suddenly solving the whole thing given only 40% or 50% of the letters" effect can be kind of cool, or annoying if you were enjoying the logic puzzle aspect. In the original MIT Mystery Hunt hexagonal regular expression crossword, most of the grid did not spell anything recognizable, so it was clearly a logic puzzle through-and-through. I guess whether people appreciate that depends on whether they were expecting a pure logic puzzle or a combined logic+word puzzle.