The traditional use of decanting is to leave sediment in the bottle of wine. Since the majority of wines today are sterile filtered and consumed young, there is little need for this step.
The secondary benefit of decanting a fine wine, and the primary use of it today is to increase oxygenation of the wine by exposing as much of the surface area of the wine to air. The act of pouring the wine into the neck of the decanter exposes most of the wine to air. Then a wide-base decanter can increase the surface area of the wine exposed to air.
I use a decanter like this one:
A 750ml just reaches the very widest point to maximize surface area. This decanter is only $35 on Wine Enthusiast.com; you can throw more money at it if you want but it is not necessary.
Our wines really benefits from an hour of air exposure - it goes from great to fantastic. We advise our customers to open the bottle an hour before dinner, take a sip and remember the taste. Decant the wine OR just pour it into glasses and see the difference - it's amazing.
So you don't NEED a decanter, but some wines really benefit from the air exposure. I like the idea of an Erlenmeyer flask and will have to get one!
One thing about the venturi aerators is they really introduce rather violent turbulence into the wine. I wonder if it might break apart long protein chains (similar to post-bottling bottle shock we see in Pinot) that take time to reassemble in an undisturbed bottle. I have one but rarely have used it.