I think I would rather have an assassin, rather than the witch.
I figure, you got all those pyros why wouldn't you want to bomb?
Also, it would give you a better chance at surviving the long term.

Because think about it, if they kill your scout for say... A knight and witch you would probably be close to done.
The 2 knights, especially same side while in conjunction with a BW, would be able to kill your magic fairly easily. Thus, not leaving you with much to kill a chanty and any surviving units.

An assassin would be a good way to take either an early advantage, or be that killing blow at the end where they just dont stand a chance.

the scout is well projected though. I mean, it can retreat, and even in doing so you are not sacrificing much range. For instance, if I knight were to attack the scout, it would get hit by LW, pyro, etc. All unblockable damage, completely risk free, and even then, the knight needs the scout frontal a 2nd time to kill.

In addition, if you retreat the scout to the back lines, the opponent's frost golems etc. will still not be too threatening, with all the range provided by the pyros and witch. And, its easy to say that a same side lw should be the first target (if not scout), and there is more than enough fire power to take it down.

I played Girsh when he used this form, it isn't easy to attack, and even harder to defend. Luckily, I had two clerics for most of the game, so I won. What you say about the assassin has some truth to it. It's always a possible swap. The main thing to remember is that the damage racks up fast. Even if you hit a pyro with your knight, you may end up losing the knight (its easy to chase a wounded unit with pyros).

I'm not exactly saying that the scout is vunerable, but you have to deal with the fact there is always a chance he can die.
And if you are outplayed in the beginning, and lose your scout, with that form, it would be easier to recover with an assassin.

I played a game against a guy with 2 witches, both lined up with the scout and i still kept him alive long enough to take the cleric. I could have been in a lot of trouble if the other dude had a chanty though.