descriptive programming in QTP means whenever qtp facing
dificulty with obj identification then we will go with
regular exp or desc prog .

without using the objectrepository, and by using the 
objectspy we will write the DESCRIPTIVE programming .
when ever we will go to DESCRIPTIVE programming :-
1.when ever objects are dynamicaly changing.
2.when ever QTP is not going to recognise the objects.
3.when ever objectrepository size is huge automatically QTP 
wil get down slow.
4.when ever not yet develop the build then only we will use
  DESCRIPTIVE programming .
Yes we can test the application completely without using
DP.hera we can use Object reposotary.