Genetic Engineering Q: PCR and recombinant DNA?

[radioinahole]

In genetic engineering, when we want to clone a gene, I have taken in class that we don't se PCR all the time because to use it, ou have to know the sequence of the annealing sites. I don't understand this because when you want to cut a plasmid to use as a vector, should we know the sequence anyway, to make sure we dont cut a gene or insert the foreign dna between a gene hat is important for the plasmid to actually work? For example, M13 phage has 10 genes and we shouldnt cut any of those genes otherwise we cant use it as a vector, right? So dont we know the sequence, so why cant we always use PCR?