The extra interest is only useful when we have a considerable amount of satoshis left in our freebitco.in account.
But besides that, more number of FUN tokens directly gives us more WOF spins which directly gives us more rewards.
So my aim is mostly towards accumulating more rewards.
I recently withdraw a nice amount but before that I had about 0.1 BTC on freebitco.in so the extra interest you got, thanks to the premium membership, was a nice amount and it added up pretty quickly. As you mention, you also get more daily WOF spins which is why I think 12 500 tokens is the sweet spot.