Jason, the focus isn't on the tree, but on the birds in the tree and what damage they can do.
If you continue to read Matthew 13, Jesus goes on to explain the parables in detail. I also think He sums them up rather nicely in the last parable:
Mat 13:33 Another parable spake he unto them;
The kingdom of heaven is like unto leaven, which a woman took, and hid in three measures of meal, till the whole was leavened.
In scripture, just like foul, leaven is not always a good thing. :yes Think of the birds as the enemy. ;)
Mat 13:37 He answered and said unto them, He that soweth the good seed is the Son of man;
Mat 13:38 The field is the world; the good seed are the children of the kingdom; but the tares are the children of the wicked one;
Mat 13:39 The enemy that sowed them is the devil; the harvest is the end of the world; and the reapers are the angels.
The birds sow the seeds in an disorderly fashion when they poop wherever they please. They do it in the rocks, they do it in the sand, they do it it dry, hot, dusty places. They don't really care where they deposit the seeds nor do they care if they grow or not.
jhis is what the Mustard seed parable has to say about birds ;Which indeed is the least of all seeds: but when it is grown, it is the greatest among herbs, and becometh a tree, so that the birds of the air come and lodge in the branches thereof.
there is no mention of any ohter activity certainly not sowing the seed, you're really putting a stretch on here. Leavan was used in Levitial sacrfices so to say ' its not always a good thing' implies something contray to it use here and elswhere in scripture.