<p>A few guys on our team are of the opinion that every web page in the application should be a web user control. So you'll have all of your html + event handling in the Customer.ascx, for example, and there will be a corresponding Customer.aspx page that contains Customer.ascx control. </p>

<p>These are their arguments: </p>

<ol>
<li>This practice promotes versatility, portability, and re-usability. </li>
<li>Even if the page is not re-used right now, it might be in a future. </li>
<li>Customer page might need to move to a different location or renamed sometimes in a future and moving user controls is easier. </li>
<li>This is a recommendation by MS for new development.</li>
</ol>

<p>Is this really a recommendation for new development? Are there any drawbacks to this strategy? I agree that it's nice to have a user control on hand if the need arises, but it seems to be an overkill to do this to the entire application "just in case we need it later". </p>

Seriously complicates client-side script as the NamingContainer jiggery will prepend _ctl0 etc to everything sometimes.
I don't think MS ever recommended it.  Request links to MSDN documentation.
Typically by the time your are done implementing something, and it is sufficiently complicated, you'll find a lot of "gotchas" when ever you try to "reuse" it.   A good example is relative links
in
the user control that no longer work outside of their path.
Users don't need the ability to add/edit/delete Customers on every page.  Indeed, you start to get into caching issues if you have these types of controls on every page.  For example if on an Invoice page, you add a Customer, will the Invoice control be updated with the new Customer?  All sorts of inter-control operability issues can manifest.  These issues are hard to argue for, because of course, everyone's user control will be
perfect
, so this will never happen.  ha ha right.
See if they can come up with an example where moving/renaming a user control actually saved time, instead of making it more complicated.   Draw up an actual example and show the pros/cons of each.