Not necessary at all to use an e-core coil.
The e-core coil has a couple of benefits over a standard oil filled canister style coil.
Due to the fact that the e-core is arranged in much more of a true transformer fashion than that of the canister, it is able to maintain relatively high voltage output while also increasing a higher current output. Consider this the "best of both worlds". The ability to produce a strong, high voltage spark in high cylinder pressure applications.
The lower resistance of the e-core also means less heat and a quicker charge, perfect for CD-type ignitions.
It also doesn't get hot (at all) like a standard canister-type coil, and (despite the argument that wages on), the e-core coil can be mounted in any position without the threat of leakage (In fact, the e-core is epoxy filled, so it hasn't got anything to leak).
Here is a quote from a Corvette tech page (eek!):
Quote:
According to industry experts, coils have always had to compromise voltage output against current output. More voltage increases the initial ionization of the spark plug gap, but lowers the amount of current or heat that follows across the plug gap. When a coil is designed to produce more current, the voltage output generally suffers, which taxes the ease of ionizing the gap. In order to accomplish a combination of high voltage and current, the latest coils incorporate a special "E-core" winding design. This is an efficient design that more closely resembles the windings in a true transformer. In this system, less loss occurs during the transfer of electricity due to the closed core of the coil.
Given this technology, an "E" style coil produces spark with both high voltage and current. Another benefit is the coils run extremely cool, even at high rpm. This is due to the efficiency of the design, as well as the large laminations (Laminations are the heat sink-style "fins" on the outside of the coil body).
Link to that page:
http://www.corvettefever.com/techarticles/ignition_coil_info/index.html
A link to Summit's offering of e-core coils:
http://www.summitracing.com/search/Depar...re/?Ns=Rank|Asc
An MSD Blaster SS E-core coil:
http://www.summitracing.com/parts/MSD-8207/
And a Mallory E-Core:
http://www.summitracing.com/parts/MAA-30441/?rtype=10
Long story short: If you have the opportunity to upgrade your ignition coil to an e-core.
Hope this helps 
