The priests heal and attack. You are underestimating them.
Please go play 10 games, buy Priest "upgrade" as soon as it is available, see how many times you lose.
BTW make sure this is not against AI's! 
Yes, they are useful, but they are way overvalued in terms of gold and xp. They provide minimal healing (even on my HP stacked UB).
If they had the same staying power as even Monks it would be perhaps worth it. However, at the moment it will take you two-three auto attacks to take one out, they will still be one shotted by AOE DG attacks. They are not worth the advantage they give the other side.
If you can't see this, or find it interesting to have a trap upgrade that will lose you games, then I guess you won't change your mind. However, I get really fed up losing games when teammates buy this upgrade as soon as it pops up. If they waste their gold on reduced respawns then so be it, however, this item wrecks the game for the whole team!