I often teach AOW/Dry Suit in combination. Students can, with one more dive and a extra bucks get the dry suit certification if they so choose.
The dry suit course on its own is a minimum of 1 pool session (first) and 2 open water dives. I'll typically do 3 OW dives the first day with folks wanting both certifications: Dive 1 is the Dry Suit dive for AOW and also counts as the first OW dive for Dry Suit. Dive 2 is usually navigation. The only real complication the dry suit adds here is on your final ascent: Do you remember how and when to vent air from the suit? Dive 3 is the second dry suit dive and doesn't count toward AOW. I then go home and issue the Dry Suit certification.
The second day we do the deep dive first, then usually Search and Recovery. The deep dive adds a bit of complication with the dry suit: There is more air in your suit that has to be dumped on the ascent because you're coming up from depth. No different than dumping air from your BCD on an ascent in that respect, just a different way of dumping. Search and Recovery does add a lift bag exercise that requires a controlled ascent while controlling air in your BCD, your suit, and the lift bag. This sounds theoretically hard, but by the time my students have done 4 dives in a dry suit, this 5th dive is not problematic for them. The last dive is a night dive. We mostly go dive and play with seals that use our lights to hunt. (OK, there's a nav out and back involved, and altered techniques for night diving.) Again, by dive 6 the dry suit just isn't a problem for folks.
Now here are reasons NOT to do this:
1. It's more expensive. I'm all about saving money if you can. Maybe you'll never need the formal dry suit certification to do a tourist dive. (Does anybody know if, e.g., the folks in Scapa Flow require Dry Suit certification to rent a dry suit? )
2. You really haven't figured out buoyancy control with a BCD yet. In that case, you're really not ready to play with a dry suit. However, a pool session can usually figure out if you're ready or not.
3. It's making you nervous to the point it's a distraction. That makes the dive dangerous for everybody.