The effectiveness of the best conventional therapies affects the amount of evidence you require to accept either an alternative or a complementary therapy (See "Alternative/Complementary" for a discussion of alternative vs. complementary use of unconventional therapies). Generally the more effective the conventional therapy is the more conservative you need to be about either alternative or complementary therapies, and the less effective the best conventional therapy the more it makes sense to gamble on things which are less certain and less well documented.
Conventional therapy varies from extremely effective to essentially useless depending on the exact situation, so it is absolutely essential to be fully informed about the best conventional therapy before making any decision which risks losing the benefits of conventional therapy.
Conventional therapies are almost always better documented than alternative ones so the evidence is much more solid. I think that even a small proven chance of long term survival is worth a lot compared to more questionable claims for higher rates of cure. Things very often look better before all the evidence is in. The greatest tragedies in the use of alternative therapies I have seen involved accepting a questionable alternative when proven potentially curative conventional therapy was available. On the other hand, for many advanced solid tumors conventional therapies simply don't work and patients are certainly justified in looking elsewhere.