Why can't I spend my reserve XRP and delete my address from the network? I understand the Ripple reserve system. You need some XRP in order for you to preserve your account into the Ripple network and additional XRP to assign trust line to your account. I also understand that you cannot spend this reserve because otherwise your account will be removed from the network and forgotten.

However, what if I want to recover the XRP from an account because I don't need the account anymore? Shouldn't it be possible to spend the reserve XRP on your own risk?
The issue is that for safety reasons Ripple currently has no way to remove the account root ledger node to completely remove a Ripple account. Since the reserves exist to help limit the size of the current ledger, any un-deletable ledger node requires a non-refundable XRP reserve (although if the XRP reserves are lowered, as they have been in the past, the difference becomes spendable again). Note that a Ripple account can use all of their XRP in transaction fees, just not in payments or trades.

There is a good discussion on why Ripple doesn't allow for account deletion on the Ripple forums. Basically, doing so would make it possible for a user (or client) to in error re-create the same Ripple account/address (e.g. if someone uses an old Ripple secret to "recover" their account). That would make old transactions signed by that account now be potentially valid against this "new" account (since Ripple uses a simple sequence number per account to prevent replays).