Hitting the I-15 makes sense as a way to cripple the NCR. Though looking at maps some routes are never explained as being broken*, it's definitely their best route into the Vegas valley. Whereas I don't see the strategic value of Dry Wells. I assume it was just for the sake of referencing Ulysses history, or for his revenge by any other name, but it got me thinking. What would Ulysses even target to destroy the Legion? What roads might he cut? Obviously limitations of the game prevented the most sensible option: sailing down the Colorado, with any number of places to get off including Lake freaking Mead. Kind of hard to cut off a river. Seems like highway 40 and/or 93 would be their I-15. Though if they're going to conquer the NCR, they have quite a few options further south, and the problem starts to expand to more than just New Vegas. *E.g the 40 east then any number of roads north including the 95 (although the in game map has the mountain range on the southern end weirdly IIRC). Or southeast on the 95 from Broken Hills. It does say Yucca Mountain is dangerous and of course Area 51 would have been hit hard, but there are plenty of ways to get around it. As well as known town along the way, Caliente. Which is only cut off from the east as far as I can tell.