What is the impact of a buckled wheel? My rear (disc) wheel currently has a lot of movement in it. I have been riding it like this for 18 months due to intending to replace the rim (has been dented) rather than simply getting it trued. The movement has worsened but nothing else really noticeable has changed.

So I am now considering what are the impacts of ignoring a buckled wheel and continuing to ride. These must sit on a spectrum from ride comfort through to catastrophic failure but I don't understand what they are. There also in my mind be an impact on ride speed or additional energy expended by I can't quantify these. 

So simply:

What is the impact of a buckled wheel?
Depends on how extreme the wobble is.  It takes very little lack of trueness to cause rim brakes to drag, but in a pinch you can usually disconnect the cable or some such and still manage (assuming the brakes on the other wheel are OK).

After that, the wobble of wheel is distracting and will make the bike unpleasant and tiring to ride at high speed.  Also, of course, it impacts the stability of the bike, making falls more likely.

If the wobble gets worse, the wheel can begin to rub against the fork or stays, and it gets unrideable.

One does need to be wary if, say, a single spoke is broken, causing wobble.  This situation puts additional stress on other spokes, making a second failure more likely, and a second failed spoke in the wrong place could cause the wheel to become suddenly quite "loopy", resulting in a crash.