There's a series of Mulcher badges for tending patches and the Whack Job series for chopping down dead trees. Poisoning trees is the only way to get them, as far as I can tell.
YMFaery, you are correct... you don't NEED to poison to do Last Egg... BUT it is a simple way to do it that doesn't involve luck of the draw of having 4 or more friends running around with you.... BUT they'd only need to poison one and would probably hang around.
Also not that some poisoning is by mistake (speed pet/water/harvest with poison in your bags and it is easy to make a mistake.
And some folks might just enjoy poisoning trees. Which gives others the joy of planting new trees. The circle of life. Can you feel the love tonight. Hakuna matata.