Your dismissal of OHKO clause isn't really on the money at all. My post here: http://www.smogon.com/forums/showpos...42&postcount=2
goes into some depth about the viability of OHKO moves in practice. Dugtrio may gain a niche at taking out support Pokemon it previously could not, such as Chansey and Blissey (51% to KO for two tries). Lapras is extremely bulky and has Water Absorb. I could go on, but there are definite niches to the usage of the move, the main being dominating slower defensive teams which rely mainly on residual damage and phazing to win. Defensive approaches to dealing with Pokemon having one of these moves in their arsenal, such as using Toxic or chipping away at a Pokemon's HP give a large window of opportunity to land an OHKO move, and the chance is actually in favour of the OHKO user.
Jibaku's post explains some of the foreseeable problems with using evasion boosting moves. The most important thing to take away from it is opportunity cost and movepool limitation. Dedicating one slot to a situational move (and arguably much more situational than an OHKO move), in addition to using turns where one could do things with much larger utility, such as setting up Stealth Rock or a Dragon Dance, would often prove to be the wiser move. I will not deny outright that there are possibilities to the usefulness of this move, as you said, perhaps with Baton Pass the opportunities to abuse evasion may prove to be troublesome. However, I doubt they would be gamebreaking, as specialization can and will occur to handle it if it becomes out of hand. Again, as Jibaku rightly said, Perish Song is a hard stop to this strategy, and there will be a few turns where one has to rely on luck not to get phazed out or KOed.
As I explained in my previous post sleep abuse would be annoying, but is far from completely unmanageable, a team of 6 Darkrai's (should Sleep Clause be removed) could easily be swept by Stealth Rock + Lum Berry Extreme Killer, just to give an example. One forgets that distribution of sleep is limited to basically just Darkrai, a frail, not overly powerful threat (80 BP STAB move) that has several established strong checks, Manaphy, RestTalk Kyogre (RestTalk Ho-Oh? Could be a thing). I believe the tier has the suitable tools to adapt to the removal of sleep clause, as annoying as it might seem at first.