I think a better whale question would be “should I transfer my BTC to ZBTC so I can gain some privacy?” How about my ETH to ZETH?
As a whale 
I can take a dip in the Zcash private pools to gain privacy for my coins and take advantage of Zcashs much lower fee structure to transact with my new ZBTC before I convert it back to BTC sometime in the future.
Is there a need for UDAs to somehow use real ZEC to pay transaction fees? Thereby the more transactions using Zcash UDAs the more Zcash would be utilized?
The big value proposition I see in UDAs is not necessarily the current ZEC holders using the assets to convert ZEC to a UDA, they already have the privacy benefits. It’s attracting other coins holders to use Zcash assets.
And the additional benefit, as @decentralistdan points out is stronger privacy for all Zcash users because of more transactions moving in the shielded pool.