It depends... on the abundance of food, on your bot, if you're running an evo sim or a league... lots of stuff...
I usualy use tiefeeding with small bots (Using the counter leech trick, forcing the oponent to try to leech you while you counter it).
So in that case it's better to only take energy since you can stay small while feeding. Locust grows according to population size and actualy starts off using only -1 but once a certain population is reached it starts to drain nrg down to about 1200 energy and then switches to -6, to try and make the most of the alge (And it's a fast way of increasing the average bot size)
But it can be usefull in other ways, if a large bot is attacked by a tie from a small bot you can eat it's body real fast (I think)...
Anyway it's quite usefull... but I think this is mentioned on the sysvars page... maybe not... should be
You can also share other things than energy and such... anyway the sysvar and operator pages often have good tips and such, I know a lot of links don't say much, but some of them do have some extra information that can be worth checking out...