CBSE 12th

One thing you need to clear here is that not all the good colleges conduct entrance exams. Some also admit students on basis of Merit in XII only.
Other than that, if a college conducts a entrance exam so it is not like it will admit you on the basis of entrance test only. It will give weightage to your past academic records also.
For example, admission criteria may be:-
Weightage to XII Marks - 60%
Weightage to Ent. Test. - 40%
( or vice versa and any other pattern can also apply )
So it will total up to be 100%.
*And the Group Discussions & Personal Interview Also Add Up To Make A 100% in Top Colleges!"
No institute admit students on the basis of Entrance Test only, it gives due consideration to XII marks as well. So here lies your answer - Yes XII marks are of much value for admissions.
Say for example 'A' scored less in XII (85%) and in E.T. he scored 90(out of 100).
Another person 'B' who scored 95% in XII and scored 90 in E.T.
So being a Selector whom would you select?
Of course, you will select B who has more sound past academic record.
So, I think now your doubt is clear.
Got more queries? Kindly revert.