The Warlock is a great soul for nearly any mage to have as a secondary soul purely for the bonuses. You could build yourself an Elemental Summoner primary build and dump the rest of your points into Warlock for the passive bonuses. http://rift.zam.com/en/stc.html?t=0zvq_.xx0V0VV.ExMt0Vttz
Just an example.
If you're greedy you can move 5/5 Improved exposure into 5/5 Dark Power, sacrificing 5% increased raid-wide magic damage at the gain of 10% increased personal magic damage, a difference of only 5% for yourself anyway, but you get the gist of it.
Essentially, you' be spamming mostly Crystalline Missiles and using Lightning Strike on cool down, while maintaining Life Leech and Dark Touch from the Warlock tree. Every bit of theory crafting I do proves that Ignite is an exceptionally weak dot that you should probably only cast if you're moving and can't cast anything else (Crystalline Missiles does more DPS per cast time at 5/5 Velocity.
I'd honestly recommend Stormcaller if you want to go as an Elemental Summoner, honestly. http://rift.zam.com/en/stc.html?t=0zyqs.V0tx0xcsb.VxMt0Vttz
for Elemental Summoner as your Primary, or: http://rift.zam.com/en/stc.html?t=0zqys.VhMt0Vtxz.V0txMxcsbo
for a more Stormcaller focused build, even though it's a 33/33 split.
Again, these are just rough builds I'm linking to give you a general idea.
Personally, I think if you want a pet class, but want DoTs and a use the Warlock soul, then stick with a Necromancer/Warlock for the great synnergy between +DoT damage and +Dead Damage, etc...
By level 12, the Necromancer gets an actual Tank Pet similar to the Earth Elemental, and at 21, you have access to upgraded versions of your melee DPS pet and your Tank pet, and by level 26, you gain access to a caster pet, so it has just as much diversity with it's pets as an Elemental Summoner would.
But it's your choice.
All Characters on Aedraxis - Defiant-side.
Neuroticaine - Level 19 Cleric (Shaman)
Rhayge - Level 10 Warrior (Reaver)
Veyne - Level 10 Mage (Archon)
Sevrys - Level 15 Rogue (Assassin)