Possible reasons you might want national validations of a European Patent
Flexibility is paramount
The national validations of a European Patent offer you the opportunity to allow the patent to lapse in certain countries while remaining in force in other countries. The Unitary Patent is an “all or nothing” patent for which you will have to pay a renewal fee (equivalent to the rate for 4 countries) for every year in which you want the patent to remain in force.
A Unitary Patent may be more expensive than a modest national validation programme
If validation in one or two countries provides sufficient coverage, the Unitary Patent is likely to be more expensive in terms of renewal fees and validation costs; validating a European Patent nationally in certain countries, such as the UK, France and Germany, does not require any translations beyond the claims translations needed for grant (which are also needed for a Unitary Patent). The Unitary Patent requires one full translation into a second European language.
Nationally Validated Patents can be litigated nationally or at the UPC
You may wish to avoid the jurisdiction of the UPC, for example to avoid the threat of central revocation by the UPC. This can only be done by securing national validations of a European Patent and opting out of the jurisdiction of the UPC. Even for such national validations of a European Patent, litigation at the UPC is a possibility at the discretion of the patentee, by withdrawing an opt out (so long as no national proceedings have commenced). It is not possible for a Unitary Patent to be opted out of the jurisdiction of the UPC.
Flexibility with regard to licensing and royalties may be important
Having different patents in different European countries may offer some advantages when it comes to licensing to different parties in different countries. If a Unitary Patent is licenced to different parties in different countries, it would be prudent to include in any license agreement a clause relating to which parties can bring an enforcement action and thereby risk a counterclaim for invalidity (which would be effective for all countries) in a UPC infringement action. Agreement on such a clause may be difficult to reach.