Because it goes against the spirit of the competition- you want your best players to be playing in the bigger games. This was probably Liverpool's marquee fixture for the group stages.
Also, another reason why people are criticising Rodgers is because he talks pretty big about Liverpool and their European pedigree. Playing a "B" team against the defending champions indicates that you have a clear inferiority complex, regardless of how he wants to dress it up. Lest we forget how Mourinho was criticised for his decision to field a weakened team against Liverpool in April.
I respectfully disagree.
Firstly, he didn't play our under 18 team. Moreno, Manq, Allen, Migs and Lallana are first team regulars. Secondly, if he had played a full team last night, only to play a weakened team against Chelsea, what would you have said? We knew it would be tough to balance all competitions and we know we don't have a strong enough team.