My thoughts on it:
- Moves guaranteed to hit are few and far between, and most are shit and do dick all damage, especially if they're non-STAB against neutral targets. The only exception is probably aura sphere (respectable 80 bp); the rest hit for 60 or below. Some teams might carry one or two of these moves on a pokemon for when the connection is needed, but if evasion was allowed then every mon would need one meaning a slot it used up for a substandard move.
- Evasion is ridiculously spammable; guaranteed hit moves are not as they're carried by only some pokemon and hit for shitty damage as mentioned.
- If evasion is allowed, battle strategy will more or less be centralised around getting a strong attacker at +6 evasion as it more or less guarantees a win, especially if the sweeper has access to recovery or if in e.g. doubles you could get a heal pulse cleric and a sweeper at +6. To compound it, if both teams do this then the luck factor in winning matches becomes too large which is part of the reason sand veil got banned on garchomp.
- Because not all pokemon carry moves that guarantee a connection, many pokemon would become immediately non-viable for usage.
- Spamming double team/minimize is a fucking sweaty-assed strategy that no-one enjoys playing against.
