Here's a good way to look at siping:
1. It increases grip in all situations.
2. They have to be designed into the lugs, or they weaken the lug. Most good sipes don't cut all the way from side to side on a lug for the reason of strength. Tires that have been siped after the fact usually have sipes that are full width or length of the lug and therefore weaken it too much. Note the picture from above and see that the sipes are not cut all the way through except for a few in the INNER lugs. The outers are not.
3. Unsiped tires on a daily driver are a nightmare waiting to happen. I've slid off the road on my unsiped muds and replaced them with Mickey Thompson MTZ's for that reason. Much better in the rain.
4. Unsiped tires are good in deep stuff: mud snow dirt etc. Siping allows them to be better on flatter surfaces especially those with a thin layer of snow, water and they're even better on ice than unsiped tires (though nothing is great on ice other than studs and things like that).
I don't think I would sipe tires that had to spin in rocks and so forth by having them siped after manufacture. If they weren't designed with sipes, probably better off without them if you bang on your tires a lot.
Here's how the sipes are done on my MTZ's...