Going out of Carling cup early was great for Liverpool last season, specially during Christmas time. They could focus 100% on PL.
If you go through, you play:
One match in September
One match in November
One match before christmas, in the crazy time.
Two matches in January (City played 9 matches in January last year)
Then final in March.
Ridiculous amount of matches for a cup few care about.
All the big clubs play their kids and reserves anyway up until the finals. I very much doubt those matches make a difference.