Light is also extremely necessary! Especially against Dugald (mass evasion!!!! :eek:). Resurrection and Antimagic are also great spells. But Light doesn't have as immediate an effect as Summoning, especially at the beginning, and I've noticed this often. On large maps, you have to sacrifice something; I usually sacrifice Enchantment or Light :( and replace it with Logistics. However, you can't completely ignore Light. It would be good to collect the complete set of Gnome Armor – it provides mass evasion and Stone Skin at higher levels. If you decide to develop Light and are planning to create a White Mage, you need to invest in the corresponding perks.
Added 5 minutes later
Light is still worth taking; sacrifice Enchantment or Education (although it's definitely needed on XXL maps!)