Those are two separate issues, dialerstring. The static route you mention gives the router a default route pointing to the dialer, but it weights it very poorly. I'm guessing that there is another default route in your routing table with a lower AD that is your primary route. If the primary goes away, the backup default route takes its place.
In the case of the summary statement, that instructs the router not to advertise all of the smaller prefixes to the peer when the dialer interface is active. Instead, it will advertise a single summary entry. I don't know why they AD has been changed from default on the summary entry. Perhaps the remote peer has another active route back to the network and the higher AD will help ensure that other routers in the network don't try to send traffic over that dial up link unnecessarily.
