Keep in mind that Clam's "edges of level" and bombsite's "edges of level" are not quite the same thing. For example based on how bombsite's scrolling works, he's thinking of the logical edges of level, beyond which there is nothing but blackness (and so in his version, it won't even let you scroll past those logical boundaries, which is, well, logical and sensible). However, Clam's thinking is of the DOS version's concept of level boundaries, where the level area has a constant size (about 1580x160 in DOS) even if most levels don't make full use of the entire area, and edge effects only come into play at the edges of the full 1580-pixel width.
I'm personally open to all intepretations of edges and edge effects. It is true though that since the DOS version's "edges" are typically far away from the level's main areas of activity, you typically can't make use of edge effects to your benefit. Also, obviously turnaround edge effects are more advantageous to the player than edge effects that kill lemmings. So if you have the edges of levels actually match the real extent of the level like bombsite does, but also implement turnaround edge effects, then you are potentially opening new ways of solving the levels that were never intended by the original level designers. Thus there is some argument supporting Clam's suggestion to just kill off lemmings that have moved past the level edges.