The attached document may show a situation where such behavior is desirable
where the object is part of the sentence

When a sentence containing objects with alternative text is copied to notepad,
the alternative text is omitted. The behavior is seen in object(formulas) and
pictures. 

Steps to reproduce
* Open openoffice writer
* Create a sentence as follows 
  "How to make combination of m out of n items"
  Select m  
  Click on menu Insert->object->formula
  This converts m into a formula
  Right click the formula object, 
  click on object
  click on options
  In alternative text put say m
* and/or 
  create a sentence and anchor a picture as a character in the sentence
  Right click on picture
  Click on type
  In anchor, select anchor as character 
  Click on options
  Insert some alternative text that describes the figure

* copy either sentence into notepad
  the alternative text does not appear.
* make some headings in this way 
  Create a table/index
  the table/index does not show the alternative text.

Select some text which includes an object, like a math text formula

I am unsure myself what the behavior should be because in most cases there may
be situations when copying the alternative text may be undesirable.

Maybe a check box in the options tab in the object dialog box, could indicate if
the alternative text is to be included in a plain text copy.