Hey Marvin,
For most adults, Halloween is just a fun holiday, and most of the events revolve around drinking. Any excuse to get drunk, is simply that. Furthermore it's fun to dress up and see what other people are dressing up as, etc. I dressed up as Lando Calrissian (at work, going out afterwards).
http://ta-nehisicoates.theatlantic.com/how-031008-lando_calrissian.jpg
But I made my own costume, so I didn't spend $40+ on some ridiculous Ghostbusters plastic thing. It's just about the fun.
Speaking about the candy, most people usually trick or treat in their own neighborhoods, and kids never go by themselves, unless they've reached a certain age. There are some houses that won't even give candy to people they consider "too old to be trick-or-treating". That being said, if you trust your neighbors and have a sense of community, there's no real reason to be frightened of the candy. Furthermore, the candy is always wrapped (well, let's say almost for the sake of argument), so there is still a chance of doctoring the candy, but this is a fairly rare occurrence.
In terms of pedophiles and other dangerous people, again, you probably shouldn't be trick-or-treating by yourself, or without a parent if you're young. This is very much a group activity (even if it's just an only child and a parent), and the parents don't go to the door, so if there is a problem, it's unlikely that everyone present would be in danger such that no one could run for help. I guess it is still a bit dangerous, but this is such a long tradition, and the chance of anything bad happening is so slim, that worrying about it is just an undue stress. Furthermore, with the pedophile issue, in many places, convicted sex offenders are not allowed to give out candy, and anyone can go online to find out where convicted sex offenders live. And, if you're trick-or-treating in your neighborhood, sex offenders have to introduce themselves, so you should definitely know where they live.
Lastly, with the corporate issue, how is this any different from just about any other holiday? Even Christmas has been disgustingly commodified, and several others (Mother's Day, Father's Day, etc.) were only created to sell Hallmark cards and sell merchandise for presents. It's just a recognition of people or religions in these cases, and Halloween is just fun. Why not?