Sadly I have heard of a JTS engine requiring a replacement camshaft before - at 60,000km - so the garage may well be right. And, now is the time to replace it if it needs replacing, since it is little additional labour on top of the work to change the cambelt (just a huge parts price, which you may be able to reduce if someone here can help or advise... sorry I don't know the best place to get one).
If they are willing, go in and let them show you the camshaft - it has the pointed, not-round, cam shapes (like rounded triangles) that push down the valves (sorry if this is offendingly basic!
) and you can easily see if some of the cams are less pointy than the others. As in, some of them may be worn; smoothed-over (with a bright, scratched metal finish) so the cam is almost a round circle. That means it's stuffed. The not-worn cams are usually a mostly dark grey colour, with smooth, polished surfaces.
I have yet to see a camshaft where all the cams have worn at once and I think that would be very unlikely, so I'm pretty sure you'll be able to see a difference where some of them have worn and not others (assuming that the garage is correct). We aren't talking about a tenth of a millimetre wear... more like 4 or 5 millimetres... the surface of the camshaft is hardened, so once it wears through that, it wears through the rest of the cam very quickly.
I wouldn't have thought oil leaks would be the problem, but definitely a gradual loss of maximum power that you may not notice, and maybe rough idling. Better to be fixed, and will need an oil flush/filter change as well, because the worn-off metal has gone into the oil.
See Camshaft - Wikipedia, the free encyclopedia
, though note that the Alfa JTS cams are a lot more pointy than those in the photo halfway down the page, due to the way the Alfa cams operate the valves directly, without rockers (levers). The animation at the bottom of the page is useful, because that is a double-overhead-cam engine like the Alfa JTS is. So you actually have two camshafts, but I bet they're talking about the 'inlet' camshaft (the one that operates the inlet valves).
Incidentally, in the JTS/TS and many other engines, the cams push down on hydraulic lifters (also known as tappets). These require no adjustment, as oil flows in (hence, hydraulic) to fill the gap between valve and cam. In other, usually older, engines, the tappets were a fixed size. Why is this relevant? Well, only that the hydraulic lifter probably self-adjusts to the worn camshaft, meaning less increase in noise than you would get with an engine that didn't have hydraulic lifters. Therefore, you probably didn't notice any increased noise as a result of the cam wear (some people might have told you that it can't be worn out or it would be making a horrible clacking noise.)