Well, you don't really need CE, but it does increase your skilling speed quite a bit. You probably don't like it because you aren't spamming it, the radius sucks, and monsters are probably getting plowed by your army in Normal difficulty.
In Hell, a maxed out CE clears rooms full of monsters faster than your Skeletons can. Quite possibly the best PvM skill the Necromancer has.
it can also depend on your gear. if you play single player, you may not have super high lvl minions, so CE can kill things quick. but if you party frequently on ladder, for example, you may have more +skills, so your minions could be stronger. also, if you party with several people, corpse explosion will lose effectiveness.
after skeletons it's your best friend. when the range gets up to a screen, you are going to wonder how you lived without it. once a single monster drops, the whole window dies.
I would say before skeletons actually. You skellies never make any kills anyways, its always my infinity merc that gives me the corpse and then I CE the rest. The skellies are just tanks to provide protection for you and your merc and only vs Baal do they actually help with killing speed.
If you can afford it (or on L), I would try to put an Infinity on merc and then make Insight IG. I am on NL and use Insight on my Merc. Let me tell you, when I spam CE....it clears everything! I say that the skellies are just as important as CE. They help each other. Skellies give you a shield to do CE while they lower the health to make CE effective. If I had to choose one though, I'd choose CE.
Here's what you do: save your points (after you max RS and SM) until you reach Hell. You can do that easy enough with just your merc and skellies. Then try out the killing speed. You will probably find that you can kill everything (using Amp), but that it is VERY slow compared with other characters (say: dual-elemental sorcerers). Then put a few points in CE and see what happens. With decent gear you will immediately have a good radius. But IMO the most effective Summoner has at least 10 hard points in CE, so it can clear out rooms in 1-2 blasts.