There are a number of studies that look at the effects of supplementing of otherwise breastfed babies, with water or formula. To pick one, in 1991, when 407 babies were assigned to either supplementation with water, or not. This was in Norway, where there is a lot of bf anyway, and it was in the maternity ward. The supplemented babies were less likely to breastfeed exclusively after this, and they were breastfed at all for a shorter time (you can read the details in Acta Obstet Gynecol Scand 70: 205-209).
There are plenty of studies looking at routines and schedules and timed feeds, as well as supplements (not always water, sometimes glucose water, and often formula). There are no studies (that I know of) that show breastfeeding is prolonged in these sitiuations, though there are some that show it makes no difference, and some that show supplementing shortens overall bf duration.
You can find some of these studies in Enabling Women to Breastfeed (Stationery Offiuce, 2000). You can also find studies listed at UNICEF's Baby Friendly site, www.babyfriendly.org.uk, which has refs for the Baby friendly '10 Steps', which say 'no supplementing' with anything unless medically indicated.
I think all that can be said is what the evidence tells us is more likely to happen, and we couple that with what we know is the basic physiology. Of course GF's routines work with some babies and mothers - just like the studies that show supplementing makes no overall difference, some women will succeed, though I think there is a lot of anecdotal evidence (even here on this thread) that GF babies do not breastfeed for very long -no longer than a few months, though sometimes (not always) that is fine by the mothers and fits in with what they wanted, and I have no problem with that.. There are mothers who find the routines impossible to follow, and they and their babies suffer (at least the ones who phone me, do - yes, I am a bfc).
Of course some women will decide that bf is not for them, and things have gone so wrong, that they feel there is more to life than a struggle to bf - that's their choice!! I would support anyone in their choice, but I would never say to someone who had asked me for bf help and support, 'look, are you really sure it's worth it?' That's as bad as saying 'look, aren't you sure you should carry on, no matter what?' Good bf help and support takes its lead from the mother's own agenda.
Sorry this is so long, but I was asked!!