If farmers used herbicide-tolerant crops in isolation there probably would be some risk of tolerant weeds. But in reality farmers already rotate both crops and herbicides to minimise the chances of tolerance. So, with GM, farmers can still protect their crops but with fewer chemicals and with no added risk of tolerance.
Insect pests could become resistant to GM plants just as they do to insecticides. For this reason, where GM insect-resistant crops are grown, there are strict regulations, backed up by frequent inspections, to ensure that non-GM "refuge areas" are established within the crops to reduce or prevent the development of resistance.
J.H. Orson and J.F.Oldfield. (1999) Gene flow and the practical management of genetically modified crops in the UK. Proceedings, BCPC Symposium No 72 – Gene flow and agriculture – relevance for transgenic crops, 247-252.