Topic was observed in passing in another thread, but I thought it worth discussing in its own right.
Traditionally, the best time to dual is around level 9/10 when the first class has picked up most of its perks, but the xp/level has not yet flattened out. The 'obvious' alternatives are something like dualing from fighter, where delaying till 13th gets you an extra 1/2 attack/round an another weapon pip, or from druid who benefit from low xp/level up until 12 (and then the xp curve takes a nose-dive).
However, occasionally we hear about folks taking the original class up to a HLA or two before dualing, so I am curious to hear about good reasons for doing so. At this point, we are no worried about the downtime, but more that we see a real benefit /after/ the downtime, compared to the equivalent multiclass. If there is little difference, then the multiclass would seem the more playable way to reach your goals, and may actually be superior as it will have another 6-12 HLAs picked up during that downtime. So any comparison must allow for the multi having more HLAs, and the ability to choose from both class HLAs at each subsequent level. The latter may be less of an issue for a highly-planned character, who is in the 'correct' final class for preferred HLAs (e.g., cleric -> thief dual is more useful than thief -> cleric).
So what does a dual class buy us? It buys us a character kit from the class we dual out of. That can be a significant boost, but varies from kit to kit on how much additional benefit you get from more levels. Which kits are most compelling for a high-level dual? Swashbuckler -> Fighter is one example that comes to mind. Full fighter THAC0 and HLAs, with a further +5 from swash kit.
Also consider that while you cannot dual /into/ a kit (without mods) you can get an illusionist kit into a multi class, thanks to gnomes. Given it is unusual to dual /out/ of mage, especially once the 9th level spells are flowing, I would be very interested to hear of benefits from late specialist mage dualing that justify not being a gnome.
Finally (of the points I am aware of) there is the question of a fighter's weapon pips. Being able to take grand mastery is privilege of the dual class, not available to the multi - but this can be achieved at 9th level, and a popular bug in the game engine allowed the dualed class to continue taking weapon proficient past 2 pips, even with their non-fighter level. So while the pips are nice, I'm not sure I see much benefit past the traditional 9th/13th level switches, unless there is some 'grand master of many weapons' role that has so far eluded me. Maybe you need a specific fighter HLA (in addition to 5 pips) before dualing? But why not dual /into/ fighter if that is the case?
So for the creative character builders out there - what are the high level duals that really shine, or offer something unique?