Personally I liked the idea of including 1 mage for water/food/ports though if I already had one at cap I likely would just do 4 Warlocks.
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Mages and warlocks both have their strengths and weaknesses. For dungeon grinding it's just preference and you can do well with either group or with a combination of both.
Different story if you plan to PvP a lot. If you box mages you need all mages (plus a healer) to achieve maximum burst. If you box warlocks you need the critical mass of DoT's to overwhelm healers. They play very differently as a boxed group so mixing them will weaken your group as a whole.