I am against the wholesale use of chemicals but I use them sparingly in my own garden and, after consultation with clients, will use them in theirs.
Generally, people employ gardeners because they are no longer able to look after their garden, because they are too busy to look after their garden or because they have no interest in gardening but want somewhere to sit of a summer evening. Invariably the biggest outlay in any contracted work is the cost of labour. This is where weed killers come in.
Apart from lawncare probably the biggest use of chemicals in the garden is dedicated to weed control. Glyphosate is a none selective systemic weed killer which kills everything, dare I say it, down to the roots and it is likely to be one of the actice ingredients in most of the branded weed killers in your local garden centre. I recommend it to clients because it's a cost effective way for them to keep perennial weeds under control.
Insecticides are a different matter. Britain's bee population is under threat and the wholesale use of chemical controls in the garden is not going to help matters. Most healthy plants will withstand insect attacks and the presence of insects will encourage birds. Nature usually finds a way of striking a balance but we can help by introducing a whole range of biological controls or providing habitat for beneficial creatures. If you must use insecticides use them just after first light or late in the evening.
Anyone who uses plant protection products (PPPs) in your garden as part of their job should have attended a formal training course and hold an approved certificate of competence. Broadly two tytpes of PPP are availabe, those licenced for professional use and those for amateur use. After 26 Novewmber 2015 it will be illegal for anyone not holding a certificate of competance to purchase PPPs licenced for professional use.
Before going off and buying a product for use in your garden take a look at The Health and Safety Executive's Garden page.   Got moss on your tarmac drive? In the Plant/Area picklist select Hard Surface and for Field of Use/Action select Herbicide. Click GetResults and you'll get a list of all the products available for amateur use.
Always read the instructions on the label -