I've seen it done before. I've even seen dudes sitting on milk crates!! If you can't afford a throne, I can't see why you shouldn't give it a go. Maybe take the casters off so you don't slide all over the place though!
No way. Its cool to sit on whatever floats your boat but do remember, that its important to be stable and in a comfortable relaxed position while playing. It is very easy to do long term skeletal damage to yourself if your seat is not right. Very Very Very important piece of harware, the throne.
If you already have the computer chair and there are no chair arms to get in the way and you can put it on carpet so it doesn't roll. Go for it. It may swivel a bunch too so maybe tighten that up if possible. we all have money issues now and then and if this get you by for a while it's better than no seat.