You just called me wrong and then went on to reword my stated reason for why FotM builds pop up. So it's a little hard to take your post seriously. Adding another reason why they pop up in addition to the one I stated and you repeated is not how contradiction works. And does not prove me wrong.
You do understand the saying "exceptions don't disprove rules"? In most cases the "deep multiclassing" (mostly deep splashes of Melee class combo's) you're talking about are FotM builds (that in some cases lasted a long time due to the derth of content in the past) and in some cases the direct results of flukish class splits due to the low cap, where going pure was never a real choice in the first place. Not getting ANY perks for the last few class levels and thus taking a few levels of something else is hardly a multiclassing choice, with an legit opportunity cost. It's a FotM build that lasts until they made those few levels better or raised the cap and made the new levels worth taking.
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.
Really how is an INT specced Mechanic lower in trap DC's to a Wiz that doesn't get Mechanic PrE (+4 DC's +4 trap saves) or Rogue capstone (+2 INT)? OR Wiz Captone (-2 INT) Also let me know how a 2 splash gets improved evasion?
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".