Of course DIY is worth it.
Do you do it at home or get a professional in to do your decorating? stick a shelf up? etc..
Of course if you employ, them you need to be within your terms of insurance and employment law, so it may not be ideal. So you would get a professional in to put up a shelf in the office per se, you wouldn't do it yourself, but at home of course you would do it yourself.
But for many of the window cleaners out there that are sole traders, its a cheaper way of doing something that get the job done without breaking the bank.
Personally i like to DIY everything, if it comes to a point where i cant do it, or a feel i need professional help then i'll go and get it. That goes for anything in life not just WC.
The essense of DIY is to save money, personally i cant afford to spash out on pre made systems so i'll try DIY first, if the system works ok and i get business from it, i'd probably by an off the shelf version at a later date, i expect that most would, especially so if you expand and start to employ.
Besides not everyone is DIY inclined, you just may not have the mentality or the time to site there and work it out, you might be the type to want to have something ready made for you to plug in and go.
So yes it does have its place, but thats depending on the circumstances you are in, and the type of person that you are.