I look at poison food and poison drink as introductions to the path. And yes, plant is essential. However, I don't find them useless. I used those two skills early on along with plant to find out how well a particular ingredient worked and still use them to test new ingredients. I looked at the population of Galadon as my laboratory and everyone in it a potential lab rat. And while most do carry their food and drink in containers, if you use your noggin a bit, it becomes rather easy to succeed. An example? While most people wouldn't even consider picking up unleavened bread from the street, it's as good as gold to me.

Take a few hints from assassins. Watch your target a bit. Carry a few common food items like berry pies, boar steaks and such. Most people follow the same patterns over and over, relying on the same food sources or relying on the same type of drink container even if they do keep them hidden. My personal favorite are those that use scripts to eat and drink.

Poison thieves are not warriors, you can't run in and spam bash someone to death. Brute force just won't cut it. But, if you consider yourself more of a spy variety, watch your target, take notes, be a thief, you'll have a lot more success and find loads of uses for poison food and drink.