Try explaining both indexing behaviors to a newcomer and you'll see the difference.
Old behavior: `3:3` causes the dimension to be retained; `3` causes the dimension to be dropped if it's a 'trailing dimension' (all the later indices are also scalars) but retained if it's a 'leading dimension' (there's at least one vector dimension after this one). New behavior: `3:3` causes the dimension to be retained; `3` causes the dimension to be dropped. A lot simpler, no? If the former rule seemed better, it's surely because of habit, not because it's actually simpler.
