Add motivation for spendable tag

Author: sethforprivacy

bip-0329.mediawiki

@@ -26,8 +26,10 @@ These standards are well supported and allow users to move easily between differ
 There is, however, no defined standard to transfer any labels the user may have applied to the transactions, addresses, public keys, inputs, outputs or xpubs in their wallet.
 The UTXO model that Bitcoin uses makes these labels particularly valuable as they may indicate the source of funds, whether received externally or as a result of change from a prior transaction.
 In both cases, care must be taken when spending to avoid undesirable leaks of private information.
+
 Labels provide valuable guidance in this regard, and have even become mandatory when spending in several Bitcoin wallets.
 Allowing users to import and export their labels in a standardized way ensures that they do not experience lock-in to a particular wallet application.
+In addition, many wallets allow unspent outputs to be frozen or made unspendable within the wallet. Since this wallet-related metadata is similar to labels and not captured elsewhere, it is also included in this format.
 
 ==Rationale==