The EHIC provides you with the same cover that a Spanish citizen would get (which is fairly similar to what you'd get on the NHS here anyway).
The only really important 'extra' offered by medical insurance is the cost of repatriation, if necessary.
The vast majority of travellers don't really need medical insurance when travelling within the EU. (I certainly don't bother with it). But anyone who thinks that medical repatriation might be required ought to at least consider it.
Standard policies usually don't cover anything related to a pre-existing condition but this specialist broker might be able to find suitable cover at a reasonable price:
http://www.able2travel.com/
Chris