In reply to Michael Ryan:
Pegs are a bit of an anachronism really. On the one hand they *are* fixed gear, on the other they still have to obey the rules of the rock in that there needs to be some form of weakness, crack, seam for them to be placed - Ken Wilson's "bending a knee to the crag". The only drilled peg that I can recall is that of Gary Gibson's on The Haunted on Craig yr Ysfa.
In some instances they are essentially surrogate bolts, most commonly on limestone routes of the 1980s when bolts were still beyond the pale but the rock didn't really lend itself to wire, cams and slings. Most of these routes have been retro-bolted and TBH are probably the better for it. There is a strong argument that they should have been left but human nature being what it is ...
The remainder may be divided in to what might be called "traditional" pegs and what Ed Cleasby referred to as "biscuits" - cop-outs. Traditional pegs date from a time when there wasn't the plethora of gear that we have today, even in the 1970s gear was essentially slings and a few larger nuts, think Rock #7 as the smallest size, so the only way to protect thinner cracks was with pegs. When Hexentrics came along the need for pegs reduced still further and again when Chouinard stoppers appeared.
So by the mid to late 1970s the only pegs that were being placed were knifeblades and a few king pins. Then around 1980 RPs arrived, now the only reason left for placing pegs on new routes was as a cop-out, Cleasby's emergency "biscuit". Even the top climbers of the day didn't have modern day limestone fitness so the new routes that headed in to strenuous territory were hard work especially if you had to place gear so a peg or two whilst still accepting the nature of the rock was seen as being generally acceptable even though there might be hand placed alternatives.
So, after a bit of background, back to the OP's question. Here's my cop-out: it depends.
Some routes depend on pegs for their protection, something like One Step Beyond in Swindale has just the two pegs for protection. On the other hand some pegs have gone and no-one has really noticed them - there used to be a peg near the top of the main pitch of Astra on Pavey Ark, it snapped when someone fell on it and they unfortunately decked it. This was 1981 or so, there's a good wire placement nearby and the peg has never been replaced. Just round the corner on Gimmer the peg on Kipling Groove seems to be remarkably persistent despite there being enough placements that you could clean aid the route.
I've replaced a peg or two: Norseman on Dow Crag and some on Raven Thirlmere but to be honest it's more about delaying the inevitable decay than preserving the nature of the first or early ascents and I don't think they should be replaced any more. Without the peg something like Norseman will go from E4 to E5, this shot
http://www.ukclimbing.com/images/dbpage.php?id=161990 shows Keith clipping the peg.
Old aid routes like Trilogy on Raven Crag, Langdale are quite literally littered with pegs, someone mentioned there being 17 of them. I can't remember clipping them all so some must be dubious, split eyes and the like and some will be the actual aid pegs with the free version taking a slightly different line. It's a long while since I did the route but I definitely placed some gear (!) so maybe it needs a bit of a clean up but then who decides on which pegs should stay and which should go? Obviously the blatantly bad ones should be removed but what about those where there's a nut slot a metre above or below but which is really awkward to place and thus might blow your on-sight? You certainly don't need the pegs to show you the way!
A Welsh example might be Central Wall at Castell Cidwm, there's about half a dozen pegs around the crux and with modern gear I'm not sure any of them are necessary, there's gear at around foot level when you clip them so it's not as if it's miles away from your last piece.
What we need to be careful of is applying current ethics to the past without understanding the environment in which climbers operated then. If it was you or I climbing the route with the gear of the time would we have eschewed the peg?
Phew! A bit longer than I anticipated but there you go.