I have no issues whatsoever with players describing the visual effects of spells. The spell itself does not change and will not do anything more than what the rule descriptions say.
On that note, I also say that regardless of the spell description, if the spell ruling does not state an effect on gameplay, then there is no extra effect. There was once a discussion on Holy Aura, because it bathed the targets in holy light will that prevent skills like hiding and make the target easy to spot if they were invisible? The spell itself states no such ruling or effects, therefore none are given (for example, Glitterdust actually states "-40 to hide").
If you are facing superstitious primitive warriors who are deathly afraid of cats and you describe your magic missiles as looking like "flaming cat heads", no problem. The spell still will not have any effect other than the auto-hitting damage.