Should users be encouraged to add some random words to reduce these (admittedly minuscule) risk?
There's good reason to add
random words passphrase after 12/24 words sequence, but avoiding possibility of duplicate generated seed isn't one of them.
The 12 words are 128 bits of entropy, which is considered more then enough.
To be more specific, Electrum use cryptographic secure PRNG which provided by the OS (for example,
/dev/urandom for linux) through function
os.urandom.