Some of the walkthroughs and posts indicate that a custom starting party is always best. The logic here is that though the preconstructed classes would take more skill points to build, there are many things you would leave out in a custom build.
However, there are several preconstructed classes that one should consider over a custom character.
1. For a Mage, if you create a custom character with Intellegence 6, Endurance 4, and 5 levels in Mage, you have 10 skill points left. The Sorcerer has all of this, and 8 skill points left. This means for 2 skill points you get: 1 melee, 2 arcane lore, 1 spellcraft, and 1 nature lore. This looks like a good deal to me. Granted, you could eventually buy all of this, but what does that work out to, 5000 gold spent per skill point saved? For a mage, I definitely think using a Sorcerer is a better choice than custom.
2. For a Priest, it is less clear-cut. If you were going to train some melee skill for your priest anyway, then the preconstructed Priest is a good idea. If your custom was 5 intellegence, 5 endurance, 3 melee, and 4 priest, then for the equivalent of 1 skill point, the preconstructed Priest give you 1 thrown, 1 arcane lore, 1 spellcraft, 2 defense, and 2 first aid. However, if you were not going to train melee (for example Slith pole priest), then custom makes sense.
3. For a melee fighter, if you were going to train 4 strength, 4 dex, 5 end, and 5 melee anyway for your custom character, the Berserker gives 1 bow, 1 thrown, 2 hardi, 2 def, 2 nature lore, and 1 first aid for just 3 skill points. Definitely better than custom in my opinion.
4. For a Priest/Mage, a Hedge Wizard (or Shaman if you also want melee) makes a lot of sense as an alternative to custom. Compared to a custom with 5 intel, 5 end, 3 mage, and 3 priest, the Hedge Wizard gives 1 melee, 1 QA, 2 arcane lore, 1 spellcraft, 1 hardiness, 1 nature lore, and 1 first aid, all for only 3 skill points.
In summary, I believe the all-custom starting party is not the best choice. Most parties could benefit from 2 or more preconstructed characters. Custom characters do have their place, especially for pole weapon specialists, or pure priests with no melee or mage skills, or a dedicated archer, or some other special purpose. But most parties have a place that 1 or 2 Sorcerers, Berserkers, Priests, or Hedge Wizards could fill better than a custom character.