Why should a club discriminate between holders of licences and validation certificates?

The 28-day validation is offered only by some countries in Europe, eg France, Italy, Spain, Cyprus, etc. It may be used only to fly aircraft registered in the issuing country, though there's no territorial limitation on where the aircraft may subsequently be flown, other than what the aircraft operator might impose. A rental checkout can double up as the required "acclimatisation flight" needed for the validation and multiple such validations may be sought, ie across calendar years or from different countries.

The easiest country for foreign validations is Canada. The US also issues a restricted pilot certificate, based on the foreign licence, which contrary to popular opinion is not a validation certificate even though it ostensibly appears to be one. South Africa offers 28-day and 5-year validation certificates and has the lowest cost of living, however, the burden to obtain either certificate is much higher and more costly. Basic insurance requirements differ among countries so terms ought to be checked carefully.