As per my first post, here's an expansion of other ideas:
-The closer they are to the player, the higher chance you'll go "A spectre! Invisible sonuva..." and notice him outright.
For me, in Doom, I didn't notice spectres too often in huge areas when they were far away. With the superiour graphics of DoomRL, perhaps you'd have a better chance, but we'll just say you don't. So, the chance you can see a spectre (i.e. the chance he'll turn from different coloured "." to "c") starts a 0% and raises up to 100%, ending just one step away from you. Hence you will ALWAYS see a spectre before he can attack you.
This seems fair to me. Perhaps Eagle Eye and Cateye can give bonuses to the chance to see Spectres. Though Intuition 2 makes the bonus unnecessary anyhow.
-Make spectres nearly identical to demons, but faster, and have less health.
My idea behind this is that they run up on you pretty quick, but one or two shots kill 'em dead. Thus, if you don't notice them, it's not TOO big a deal. Perhaps ramp their damage down and up their speed even more, so they're like a mosquito. They don't hurt much, but they sure bite a lot.
Another SOLID reason that we NEED spectres in this game: AoB. We're losing out on another melee only creature!