To be fair, I don't think that's really as cut and dry as you allude to. If Rask comes back–and that's still an "if" at this juncture–we don't even know what he'll be capable of post-surgery and recovery. Rask could be better than he was last year and that wouldn't surprise me, but he's also 34 and turns 35 in the spring.
The Bruins paid a premium for Ullmark because of a weak UFA goalie crop, Buffalo wanting to retain him and they'll have security of a solid, unspectacular goalie who played well for a bad team. Agreed with your points about Swayman and how it's a good plan.
I think Rask being a question mark, as well as Swayman being a bit too green at the moment to be a full-time No. 1 is definitely a reason to have signed Ullmark.
Either way, I'm completely OK with this goalie scenario that the Bruins have created. They removed almost $3m per year with Halak and replaced it with less than $1m in Swayman, but also replaced Rask's $7m per season with Ullmark's $5m.