As I hate this thread being converted into anything else except Spain, still - alow me to retort.
I am not consusing FP with CAS. Class E in US is CAS (as everywhere else). Radio contact is not required for VFR flight in Class E. Prudent - yes (flight following). Mandatory - no.
You right - VFR FP is not mandatory either. But even if you file an FP, you don't have to talk to anyone. Gentelmen in F16s could not care less.
In US, the only purpose VFR FP serves is SAR. ATC don't even have your FP on their computers until you miss your ETA.
Apparently all that is not the case in Spain. AIP says radio contact is not required in Class E, but flight plan is, and two-way contact whenever FP is required. I guess Inquisition still alive.