Both should not sound the same, but it is indeed often the case. As an example I will take what is probably the most common occurrence of words ending in -ai, the future tense:
Je serai à l'heure au rendez-vous
J'aurai des vêtements de rechange
In these cases, it should be pronounced a bit like "é", but indeed there's a tendency to pronounce it "è". While in most cases there is no ambiguïty (people will still understand this is the future), still it is not the correct pronunciation. I also think this induces spelling mistakes as you would be tempted to add an "s" in the end (if you pronounce it the same, why not write it the same after all)
I will compare with the same verbs in the conditional tense:
Je serais bien parti en vacances, mais je n'ai pas d'argent
J'aurais bien voulu partir en vacances
In these cases, it should indeed be pronounced "è", and usually it is.
In the other direction, words that should be pronounced "è" but are pronounced "é" (poulet, lait,...), the pronunciation is depending on the region where you are. On this, I can't refrain from thinking about this (well-known) advertising:
Note: I'm a native from central Belgium and always lived there.