At the moment I think this is by far the least useful glyph. Although it can be an important part of some puzzles, and occasionally some puzzle-like situations in dungeons, as well as an early source of piety when you have nothing better to spend your mana regeneration on and of course conversion fodder, I can't think of any situation in which I've found WONAFYT where I wouldn't have been much happier to have found just about any other glyph.
- It only summons monsters of your level or lower. Most of the time I'm exploring at full HP/MP (wasting tiles) it's because I think there are higher level monsters out there that I can kill for the bonus experience. There aren't many situations in which I am in need of a lower level monster, and when I do need one they're usually plentiful, in large part because the dungeon spawns more lower level monsters than higher level monsters.
- It's expensive. At six MP, even in situations where it might be useful, it's generally not worth six tiles of regeneration. Pretty much every other spell is more cost effective.
- It's unreliable. Many of the hard dungeons have enemies that resist magic, and since WONAFYT selects its target completely randomly, it's perhaps the most likely spell to fail as a result of monster resistance (e.g., the player is smart enough not to target a Frozen Troll with WEYTWUT, if it can reasonably be avoided, but WONAFYT is just as likely to pick a high magic resist enemy as any other type). If it fails, you've just spent 6 MP for nothing.
- It doesn't add any value. If I found a same-or-lower level monster by exploring six more tiles, I would be in the exact same position as if I had used the glyph. The weaker spells from the alpha - WEYTWUT, ENDISWAL, PISORF, and IMAWAL in particular - all got secondary effects that make them useful even when you don't need their primary effect. If you don't need to summon a low level monster, there's no particular reason to cast WONAFYT.
In light of these problems, and desiring every glyph to get some love rather than going straight to the conversion pile, I think it would be worth considering a few possible changes to WONAFYT.
First, you could have it summon monster of a higher level than the player. Not only would this make the glyph incredibly more helpful for nearly every class, it would have the added advantage of making it more literal - don't use this spell unless you really want a fight.
Second, you could reduce the cost. If it's going to remain the same spell as it is currently, I'd say it's worth two MP, maybe three at the very most.
Third, you could make it ignore magic resistance, or if the first target successfully resists have it pick a second target. WONAFYT is currently the only glyph that can fail for reasons entirely outside the player's control, which is a very poor mechanic considering that it's also the least useful glyph in most circumstances.
Fourth, you could add additional effects besides summoning. It could cause the same slow status that WEYTWUT does. It could give an experience bonus to the monster that it summons - to prevent abuse, I would make it something like +2 or +25% experience but the effect wears off if you summon another monster, attack another monster, or regenerate via exploration (i.e., you have to kill whatever is summoned right away to get the bonus).
Obviously implementing all of those changes would make the glyph overpowered, but I think making one or two of them would be a significant improvement to the glyph and to the game's balance.