What are the benefits of holding onto deflationary coins for yield pool value?

There are definitely benefits of holding onto deflationary coins. Unlike inflationary assets (like the US Dollar), deflationary ones do not lose value over time. Holding your deflationary coins in a yield pool would help you earn even more deflationary tokens, which cannot lose factual value - i.e. holding your Bitcoin in a yield pool will only give you more Bitcoin.

If, for example, you have 10 Bitcoins in a yield pool that offers an APY of 2-3%, you can get up to 3 Bitcoins yearly in interest for every time someone uses the coins you've staked in the pool. Since their deflationary, you can expect their price to eventually go up, making you a lot of profit along the way (a lot more than the expected 2-3% yearly).