Wouldn't it be great if you could make something once and sell it a hundred times?
This is the basis of most software developers' business model.
Create one application for a hundred clients.
Write it once, sell it a hundred times (or rent it a hundred times - in the case of cloud applications).
This model, in itself, does not stop customisation for specific clients.
But like most applications there will eventually be upgrades, the problem with these upgrades is that they can only be applied to the standard application. If any customisation has been done, these new changes won't fit and therefore can't be upgraded.
For this reason software companies working with this model resist customising applications for their clients. Not because they don't want to help their clients but because it would mean they would need to change the way they do business.
The other main business model is..
Create one application for each client.
Include only what they want, and upgrade their application with the changes they want, only when they ask for them.
This is the model we use at nuSoftware.
In fact we go one step futher. Any code we write for you is yours, if we create an application for you, that you think you can resell, you are free to do so.
What could be more flexible than that?