DSL and cable modem users do face a greater risk of Internet attacks than do dial-up users. But the main difference between the dial-up users and those connected fulltime is one of magnitude. When you use PPP, your system becomes part of the Internet and is subject to scans and attacks. DSL and cable modem users are already facing attacks based not on the fulltime nature of the connection, but because the address ranges used by suppliers of these fulltime connections are known. If past history is any guide, attacks seeking out systems connected fulltime to the Internet will increase.
Any system connected to the Internet needs protection.
For desktop systems, software or hardware firewalls make sense, whether you use a modem or a fulltime link. PPP supports not only IP, but other protocols such as NetWare and AppleTalk. For most of us, this is not an issue as we are either not using these protocols, or are connecting to an ISP that will not route these protocols. The primary use of PPP has been to setup connections over serial lines connected by modems. PPP handles authentication, negotiates compression, monitors link reliability, and also handles breaking down a connection.