I'm not sure why it's relevant that the SBM can move forward while casting creatures...if anything, I wouldn't want him near the fighting since most of the mana is being spent on creatures and not defensive cards. I usually keep a mage that is swarming at least 3 zones away from the opponent.
Then you are letting your opponent use their melee attack while negating your own. That is 3 or 4 (or 5 for beastmaster because of pet), dice of lost damage. If your opponent attacks you instead of your relatively fragile creatures then that is great, now you get to absorb the attacks while your cheap creatures swing for 3+ dice each. Your first 10 or 15 hp are a resource that you can spend for more short term advantage. Then use that short term advantage to gain a long term benefit and you retreat to prevent your opponent from finishing you off. Now you have control of the board and just need to survive. Alexander often did this, using initiative timing to run away and start drinking from the healing fountain. It's a risky tactic that can fail if your opponent has more burst damage then you expected, but if executed properly it is very valuable.
In terms of "pit" strategies, I don't really consider them high quality. It's the same vein as wall of thorns/push cheese in my opinion; exceptional if your opponent doesn't have a tuned deck, but poor if he does.
An extreme pit or relying only on WoT+push will fail against an experienced opponent but having those tools in your spellbook can be powerful.
