#1 - if you f*** up, you don't ruin a slide that's now worth some $500+ (after all the mill work, etc)
#2 - you stand far less chance of introducing a stress riser into the slide that will eventually result in a crack (again, costing you $500+)
#3 - Putting a screw into play gives you one more place for the thing to fail, potentially ending up with a situation where that screw backing out causes the gun not to run. This will inevitably (due to Murphy's Law) occur during the match that cost you the most money to be at for the year, and will possibly result in damage to the slide, frame, or other parts in the gun.
If you're going to put it in the slide, you can eliminate #3 by drilling the hole blind, and staking the ball and spring in, just like you'd do in the racker.
1. The dovetail is the hard part of the whole thing.
2. The area you are cutting isn't in a high stress place, and you're putting a round hole in it. The dovetail is the sharp angle that would break.
3. Any of the screw based detents I've seen don't come close to the holding power of a normally install detent.