Not really - a confirmed reservation is a confirmed reservation. Carriers are getting fussier about what it takes to issue and hold one, but the principle still holds true. When you make a reservation separately from a ticket then you either need to get the ticket number in the reservation or be very clear what the deadline is by which you must have a corresponding ticket in there for it not to be autocancelled. It is comforting to have an email or online confirmation that you can print out, although in at least 20 xONE3s I have never had a problem of a reservation "disappearing".
Certainly the route being discontinued is a disadvantage. But the speed of reissue if you don't have to worry about reservations is a big advantage (at HKG & LHR I have regularly done reissues in about an hour on demand; CX's record at HKG was about 23 minutes I seem to recall). I have also found that checking in for flights with an open-dated coupon (and a confirmed reservation) never causes an issue, whereas turning up with a coupon for a different date sometimes does. Another big advantage of Open coupons is that you can ticket them to a city rather than an airport. So, for example, in the past I have switched my reservation from a JFK-LHR flight to a EWR-LHR. In principle, JFK and EWR being coterminals a coupon for JFK-LHR should be accepted on EWR-LHR, but if the coupon says NYC-LON then there is no doubt that it is valid. This is helpful when you are having trouble with availability. On another occasion I ended up flying LHR-BWI when I had been unable to get availability on LHR-IAD, but because I had seen this coming the coupon said LON-WAS and there was no problem.
I admit that I have never done an xONEn ticket on e-ticket, but I have had open-dated coupons on normal roundtrip e-tickets with no problem at all, so the systems are certainly capable of it - whether the agents know how to do it is another matter.
Basically the advantages tend to be better availability and instant confirmation, although the OW airlines are getting their act together so that they can more often instantly confirm. Also if you have status with the airline that you want to fly then you will likely get a better response from calling them directly. Unfortunately, with the way certain airlines are playing with yield management these days you now sometimes get different availability results from calling, say, CX in London or CX in Hong Kong. This is a stupid way to run a business, but I guess all we can do is play the game and if you don't succeed at one office then try another. Since when you call the airline direct you are just talking about reservations then the method of ticketing doesn't matter.
In principle, an e-ticket should enable this, but personally I am MUCH more comfortable having the ticket coupons in my hand. You are then in a much stronger position in any "discussions" that might arise at check-in. With e-tickets the airlines apparently (from tales told on FT) resort too often to saying "the computer says no". With paper tickets all you have to do is to get them to agree that there is a reservation in your name in the appropriate class on the flight and then present a valid paper coupon. The "three hour" thing really depends on where you think you will be when you need the reissue. I am lucky in that my home base is HKG and that is where I have done most of my reissues - the CX airport ticket desk has never taken much more than an hour and often less than half that. On one occasion where there was some system glitch they simply let me fly the next two segments on CX anyway and I collected the ticket on my return to HKG.

But all this comes with a big caveat: I have never tried doing a OWE on an e-ticket, most of my experience is now 2-5 years old, and I have always had status with at least one, and sometimes all of the airlines I have been flying with.