The only reason I can see is that statting up and keeping track of an animal companion that will never rise higher than level 1 wouldn't usually be worth the effort. There's no real reason not to, otherwise... but you might want to take a less combat-oriented companion for flavor, like a bird or (small) dog or something.
A neat trick- A two level dip gives you brawler's flurry, which works with normal TWF feats you get from your slayer (allowing you to progress flurry after leaving brawler). So it basically gives you something of an upgrade with restrictions (close and monk weapons...but there are plenty of good ones in the martial weapon section).
The fun part with this is that you can flurry/TWF with a single weapon. That greatly reduces your costs (which you can put into your animal companion, I suppose)
You can do things like sword and board without needing to use the board as a weapon (just grab a 1 handed monk weapon, like a butterfly sword or a 9 ring sword, which you can freely use with slayer's martial weapon proficiency). That saves you on shield related feats.
Or you can 2 hand a weapon for x1.5 power attack on every hit. Also very nice.
Overall, nice enough reasons to get brawler...and if you want to trade the level 2 brawler feat for a pet...so be it. Not that huge a loss.