Dunkah
Emperor
Why would you want to prevent the former? As for the latter, I have won countless Science victories, and have never employed it or (more importantly) felt the need to.
I was playing against America. (I generally play King

I knew that he had completed Apollo... and was in the midst of completing my last few policies for a culture win. I went to scout out what was going on over his borders. The actual Victory Progress window showed him as not having completed any parts up to this point so I felt I was safe.
When I saw he had all of the parts but one right there in the open just waiting to send them all in to his Capitol on the same turn I was stunned. (Why else would he have built them and not sent them straight away to the Capitol unless to hide his Victory Progress?)
I immediately declared war and started pummelling them... but if I hadn't seen them he would have completed all of the parts in the same turn and launched without me knowing or having a chance to stop him other than taking out his Capitol City before he landed. (I'm not even sure if that would have stopped him at that point).
Edit: The Point...
The point is that by having something that can be destroyed... like a unit or tile improvement or both. Civs have an opportunity to prevent a victory of the Science or Culture types without wiping out, or puppetting cities and taking diplomatic hits other than DOW. By preventing another Civ from winning you may even get a Diplo bonus that would counter act the hit for DOW-ing.