artois Val:
For perfomance and usability.
I have used ADODB several years in Visual Basic 6 and in ASP and I have never had any problems. I have not ever used it with PHP.
I am sure you know that ADODB has been replaced with ADO.Net, but I don't think PHP has support for ADO.Net yet although I understand they are working on it.
I haven't use ADO.Net at all.
As for PDO, I don't have any idea what that is.
I guess my main thought would be: if you have something that is working and you know it well, why would you want to switch to something you don't know and would have to take time to learn.
I would only do that if there was some substantial benefit to be gained.
Good Luck