The main problem is the badge and the snobbery attached to it. Drive a VW Golf and then jump into the SEAT Leon and I know which one you'd rather have. Same with the Altea which would be similiar to the Golf Plus. We had an Altea XL for a few days over in the UK and my dad, who is now a Merc man, loved it. The market for used Altea's wouldnt be that big either and you could say the same for the Golf Plus. I'd also take a Leon over an Altea, and if I wanted more space i'd go for the Altea XL.
SEAT always had a bigger drop in resale value compared to the other brands in the VW group with the exception of Skoda, but so do Fiat, Nissan, Citroen and Alfa Romeo for example.