Is It Wrong To Allow User Initiated DDL Commands To Facilitate An Extensible Schema

I have a client who wants to build a database with a user extensible
schema.

The client wants to model an entity with about 100 numeric attributes.
As time goes on, more numeric attributes will be added. The client
wants an admin function that will allow him to add these attributes
when needed.

My instinct is that this is wrong and that bad things will happen if
we build this. However, when I went to write up the reasons, all I
could come up with was that
* we would need to write dynamic SQL to handle the changing table
structure,
* that table locking would result from field additions
* and that it was "non-standard"

One alternative is to create a table containing field-name/value
pairs. This is ugly and breaks the ER modelling rules that tell me yo
put attributes of an item into the same row. All the same, I would
prefer this method to the first approach.

This problem does not arise on database projects that follow the
pattern of only allowing DDL commands to be issued during the initial
development phase and during subsequent change requests.

Have any of you been faced with a similar problem? What should I do?

Karen