Think about how a planter works. Even running a field cultivator over it, some seed will be falling into a bottomless pit. Ripping at an angle to your rows lets the planter provide more uniform seed depth and the percent of seed falling into a chasm is much less than if one or more rows are trying to plant seed in line with and directly over a ripper slot as is sure to happen if you rip and plant in the same direction.

And the harder the soil is in ripped areas, the more likely there is a deep open slot below the soil surface and the less likely a shallow cultivation will fill it in below the soil surface.