We, at Serica and DigitalTangible actively use unspent tx chains to allow our customers to speed their bitcoin user-experience without the need for them to wait on blockchain confirmations. These transactions are usually sequential and must happen between our customers and our marketplace of merchants and other customers. For example, a user agrees to place an order to purchase bitcoin and then make a bitcoin payment, for a product or services, with that bitcoin, should their desired price be met while they are unable to actively monitor their transaction.

We currently do not have a need to chain more than 5 unspents given our current use cases for onboarding new customers into the bitcoin ecosystem. Given this PR, we agree with its principle, since the proposal aims to limit to MAX_ANCESTORS = 25 and MAX_DESCENDANTS = 25, which we think is reasonable. We have not **yet** seen a use case for more than 25 chains of unconfirmed in our ecosystem.

However, we would like to publish our viewpoint that we wish to avoid a slippery slope of restrictions in unspents to fall from from 25 to 2 or even 0. The limits imposed should not prevent, at minimum, 5 step chains of transactions that are needed to give a customer temporary control over funds that they would otherwise not have access to unless they waited for a confirmation before conducting another transaction. In these situations, where an instant purchase must be made with customer control, that btc must be sent to a customers address and then be quickly relayed to a merchant or another party in a transaction to create a seamless experience. All of this must happen within 0 confs because our customers will not wait for a whole confirm and we do not wish to lose the opportunity to make Bitcoin more available and useful to a broader audience with higher performance demands. 

Zero confirmations, when done properly, help increase adoption of Bitcoin and make it more competitive against other forms of payments. However, we do think it's good to prevent abuse of the system with reasonable constraints for the current ecosystem of applications and wallets.