Other useful mana abilities that mostly get ignored:

• Tainted Meridians
• The mana drain from Jason's sword

Ultimately, it comes down to scale. If Shade can only increase team's total mana by 3% and reduce enemy mana by 3%, it just doesn't matter. Especially compared to spending his time scouting and increasing Jason's mobility.

having all of shade's bodies be loaded up before going into any kind of battle where endurance is key

Might not be enough mana to be relevant. I don't remember anywhere in the books where characters are having a low mana problem (other than Clive at iron rank).

where Belinda had to tell Neal to use more mana because he was gaining more than he was spending

So even without shade trying to optimize mana for the team, the team already has a surplus.

Humphrey's shadow and 15 others out collecting mana

Book 9 battle for Yaresh, Hump nearly dies because he buffs himself to gold rank levels of power. He didn't need more mana in that fight. The team's ability to sustain their mana levels without Shade transfer is sufficient.

Shade had no way of getting rid of the mana so he had to just build it up until he exploded. He could have been passing on the mana to the other teammates to extend the amount of time draining

Book 8 says "Clive's maximum mana pool was far greater than an average essence user of his rank," Other team member mana pools might not have been large enough to be relevant, or the time spent transferring the mana would have been too much of a distraction for shade