Disneyland stopped issuing them because of the rental brokers. I'm not sure of the prices when they did have them, but currently the additional cost is tiny for the fourth and fifth day. As it is, they're getting stricter about checking for ID.
If you look at the ticket window prices of the 10+ current tickets and the approximate price per day:
1-day: $125 / $125 per day.
2-day: $200 / $100 per day.
3-day: $250 / $83.34 per day.
4-day: $275 / $68.75 per day.
5-day: $290 / $58 per day.
Since they typically rent them for $80-85 per day, you can see that the longer length tickets are probably where they can make the most profit. The brokers and customers using them are violating Disney's conditions for purchase and entry into the parks, although it's only been recent that Disney has been serious about cracking down on the practice.