Screenshot doesn't show anything you're writing about :wink:
You're right that it may be convenient, however it would (1) require to show those values in the select - you could have two contacts with similar email addresses so it has to be shown to distinguish them. (2) Searching works for both (with different fields) recordsets, so it has to be specified which field is used to search for every recordset. It's done in the code. We've changed select field to be more flexible and allow to select from more than one recordset, but tasks uses the old specific field type to select just contact or company.
You can change this in code if you wish. But it won't persist over EPESI update. It's pretty simple change in file modules/CRM/Contacts/ContactsCommon_0.php, lines 244-251.
We'll work on some unification of those old fields with the new ones, but it's not a big deal right now.