Web Development

find that phpgroupware will meet thier requiremnts, as well as yours.
as the customisation / increase in capabilities is adding php scripts not altering the base os.
less likelyhood of breaking anything, and a scalable structure that is designed for rapid alteration / customisation.

How standard will the standard be ?
What's it's longevity.
How different is the data.
Essentially it's the cost of carrying extra fields fot duplicate/unnecessary fields about and multiple front ends or crappy looking single ones vs the complexity and performance degradation of a design that copes with the same clas of entity being described in potentially many different ways.
There are costs and benefits on both sides.

I?d say that, as a general rule, you (we) must always satisfy to some degree the customer's needs & wishes.

What I think would work in situations like the one you describe, is for you to let them know that data standardization is not a whim of yours but a requirement for any quality system.

Based on that bottom line, you could provide them a non-data-modifiable core-system, with much more flexibility in the add-ons (front-ends and so on).

That might put things in a mid way solution acceptablo for both sides.