This way you maintain your parent stock with the traits you value the most. You have to cull ruthlesslly, make no exceptions. It is OK to keep favorites if you have room, but don't use them in the breeding program.
Line breeding is the only way to improve stock.
If you get more than one or two worth breeding from in each generation, you are very fortunate. Consistent, uniform birds come usually from years of line breeding and dedicaton. Don't be disappointed in the first few years. Out crosses introduce so many variations that you could be starting again. If you have to introduce an out cross do so with a female, control very carfully and select the best to go back to your blood lines, only use this female once.