A lot of old builds (such as the Batman build) were a direct result of exploiting a poorly implemented (or not at all implemented) PnP rule allowing someone to have evasion in heavy armor for example. Not unlike the current "hey 10,000 Stars works with arrows" Monkchers, they should never have existed in the first place. If you can describe a class split as "it would be stupid not to do this if you want to be a good ____fill in the blank then it's not much of a choice.
Let's not pretend your brief list of exceptions is somehow proof of a thriving history of great multiclassing choices.
What you said is wrong, as in ACTUALLY wrong, as in I have just objectively proven you to be wrong. Unless you can produce about 7 DC worth of trap skills that a 18/2 Wiz can get that no Rogue has access to (so far as I can see only difference is 3 INT from Wiz enhancements, or 1.5 DC's a Rogue can't get versus the 6 DC's a Splashed Wiz can't get), and an undocumented ED improved evasion ability that gives the weaker trap DC Wiz the same survivability (minus some hit points the Wiz is giving up and a couple points of reflex save because they both have Insightful Reflex feat, but one has capstone and the other does not) and thus "better at trapping".
If however you meant to say "can be a very good trapper" you know I've already said as much about multiclass versus pure. Pure is the specialist in most cases (exceptions exist). Of course there's very little content where a 18/2 Wiz with trap gear can't do traps, we both know that and naturally that's where your defense of your incorrect statement must go. However that's not the same as "wrong an 18/2 wiz is a better trapper".