I've never done an Alfa cam belt change but isn't tensioning done by rotating the inlet cam pully on the shaft? If thats right then tipex marks can never be correct.
Tensioning is generally done by moving the tensioner (an idler pulley that pushes on the back of the belt).
With the camlocks in place, both cam pulleys are freed off (so they are free to rotate - there are no Woodruff keys) and tensioning the belt is easier.
By that I mean when actually putting the belt on any engine (but especially a V6) it can be difficult to get the belt on all the pulleys at once (and it is easy for one of them to turn by mistake, especially if the cam lobes are in an 'unstable' position as they are on a V6) - and tensioning tends to be hit-and-miss requiring several checks and readjustments - it's easier if all pulleys are free to turn, as then you can get the belt on without fussing and the tension you apply is distributed evenly rather than just on the part of the belt where the tensioner is.
Having read through this thread, I think the without-camlock Tippex proponents are meaning that if everything was timed up correctly the last time a new belt was fitted, and you put a new belt on with the pulleys in the same place as they would have been when the last new belt went on, the timing must be correct. Provided you don't free off the pulleys (and if they were correct the last time), they must be in the correct places now with the new belt fitted, assuming that the new belt has the same tooth spacing as the old did when it was new. And there is very little variation between belts, so that should be the case.
But who's to know that the timing was correct the last time if the belt has already been changed - perhaps it jumped a tooth during installation/tensioning and no-one noticed - and with the Tippex/back-to-how-it-was method you can't detect or correct that. Also, if the old belt DID stretch, the pulleys will need turning (while still fixed to the cams) ever so slightly in order to get the new belt on. And like others have said, on at least one-third of the cambelt changes I've done (on other engines that DID have timing marks), I've found the timing was off by a tooth. It happens and may have only a minor effect on performance - e.g. intake cam advanced one tooth - top end power good but knocks the edge off the low-speed torque.
So I'm in favour of camlocks myself, especially if the car has an unknown past. For the price that camlocks cost, it's worth knowing that the timing really is correct.
And by the way, on older twin-cam engines people used to fit expensive 'Vernier' cam pulleys that allowed fine adjustment of the timing - 'dialling in' the cam - the Alfa's taper-fit standard pulleys give you that advantage for only the cost of the camlocks...! So rather than a nuisance, it can be seen as an advantage?