There are a lot more line holders because PBS is more efficient and builds lines around pre-month conflicts. The down side of this is generally less flights in open time.

For example, and I am using rough figures from memory, DAL used to have around 30 reserve lines for every hundred lines (this is a ballpark estimation) and now with PBS we are shooting for around 15 per hundred. Obviously different categories are manned based on specific needs but this general trend held true for all categories.

DAL uses 85% as the ballpark number for lineholder vs reserve. It does not hold each month and if we are carrying extra pilots there will be more than 15% on reserve, but the system is made to run with 15% reserves.

So on the plus side of PBS - less Pilots on reserve.
On the down side - less open time for Pilots to choose from.

Scoop