there are hundreds of ash statues in game but they are of no use.

I thought they could be offered to the ash altairs to receive a blessing from dagoth ur (as long as he is alive)

does it make sense? if not, any other way to make them useful?

If I recall, the ash statues were tools used by the Sixth House to spread nightmares and visions, so it wouldn't make sense to offer them back. They're really supposed to be negative to the average person/non-cultist.

Sounds good to me, and I'd probably go for a blessing related to blight disease.

In LGNPC Pax Redoran ash statues cause nightmares and stuff.

the whole point of ash statues are to show Dagoth Ur's way of spreading around "soul sickness" and how he enters into the dreams of people and to be used as a way for his cultists to worship him in secret.

Yeah, I think LGNPC pretty much covers this. But otherwise, yeah I think they'd have a lot more negative effects than anything... hmm

it would be better if you made a script that causes your character to have nightmares and get buffs/de-buffs randomly based on the nightmares.

I don't think that a soul sickness/debuff feature would be very popular. From what CyranO has said, the majority of LGNPC users run the no-soulsickness optional plug-in.

The use as an offering for a blessing, however, sounds pretty good to me. I'd vote for that.