This is primarily a hold-over from the DNN 3/ASP.NET 1.1 days.

IMO - there are no cons in removing the Abstract/MustInherit modifier.  But there are not really any pros either.

Theoretically - removing the modifier removes the ability to "instantiate" the class declared in the "code-behind".  That is all.  In reality there are very few (if not no)  reasons why you would instantiate the code behind class - and not want the inherited "ascx" version of the class.  Remember that the ascx "inherits" or sub-classes the code behind, so if you instantiated the code behind directly you would get none of the html layout of the control - just the code aspects.

In DNN4/ASP.NET 2, we use the code-beside model a lot - if you use that model the code file is declared as a Partial class and is combined with the ascx file - rather than the ascx file inheriting from the code file.

Brian

You might be right - I was basing my comments on what I had heard on podcasts like Dot net Rocks.  I haven't actually looked into that aspect.