Aye, it's a rather specific example, since the pyro is the only class with a DoT. I think most weapons out damage the blutsauger in most cases of a medic vs random class fight, and when you kill the enemy their dmg output is zero, except for the sole case of a pyro that has set you ablaze :/
The blutsauger has been an awesome weapon from day one, and have gotten my sorry hiney out of trouble more than once, but I have also encountered your scenario you mention above Scatter ^^
I reckon it's even nastier now with crits enabled, but now the syringe gun also offers increased survivability instead of just more fire power, just in a more passive way.
Then again, you won't be able to benefit from the full effect of the syringe gun unless you make it out of the fight alive, so perhaps the blutsauger is still to be prefered.
Need to try it out anyhow

Of course there is the other big advantage of the Blutslaughter to consider.
+3 healths telling you when your random needle spam over walls etc has hit something and helping with the spychecking.