Problem with setting Testing Targets keeping testing case count in mind

In my 5 years of experience , i observed a very common problem when test plan is written and the application is tested with the plan . ie "Managers seeing only the Testcase count and setting the time frame for testing"

Testing Targets are normally set based on various factors . They differ based on the application under test . If it is client based urgent requirement , we will have some factors which mainly depend on the delivery date and quality . But for a product that can be released with some time delay , the factors vary . Being a product based company , we have the liberty to take more time . But the main problem i see in many managers here(even outside) is that they calculate everything based on the count of the cases . Planning based on count is good , but planning everything only on the basis of count is damn wrong . There are variety of factors that come in to picture here and some of the disadvantages are below

1. All the cases are not same . Some cases take more time to validate and some take less . So when setting a time frame , it is always better to have a bit extra time

2. Time for some extensive testing . Testing plans written exactly based on the test case count with tight schedule will not give the tester a free hand in testing . For example if a tester is asked to test an application having some 500 cases in 5 days . He will test only the testcases . He wont be thinking to crack the application . He will only try to finish of the cases and submit the reports . Although that would be suffice , it will no way enhance his skills or the product .It is always better to give some extra time and freedom to test .

3. A tight schedule might force the tester to skip the cases . If not credited or not planned well , the tester might end up being under pressure which will make him to skip some cases and hence resulting in breakages .