We get this question a lot.
"Chris Schwarz put a crochet and a leg vise on his first Roubo bench, why don't you include one in your plans?"
Chris' first bench was largely an experiment. He built it to prove out Roubo's design in a modern setting. The rest as they say, is history. We were of course influenced by Chris' work, but our bench is Plate 11 at its core. In "L'Art du Menuisier" we don't have an illustration of the bench with the leg vise installed (discounting pl. 279--which incidentally has no crochet), but only close-ups of the leg vise in Plate 11. Roubo doesn't specifically say that benches with leg vises shouldn't have a crochet. But he does specifically mention the superiority of the leg vise for furniture makers. So we put two and two together and didn't include a crochet in our plans. If you do some research you'll find that anyone who's used a crochet, then a leg vise, never goes back to using the crochet (Schwarz included.)
To boil it down, don't include a crochet if you're planning to install a leg vise. There are reasons why it would not only be superfluous, but also a disadvantage. A crochet reduces the capacity of the bench (see pic above).
Although Roubo doesn't show a bench sans crochet, there are plenty of French shops with Plate 11-style benches with leg vises and no crochet.