Is Acid rain -that- bad? At first glance it would look to be a slightly weaker (at level. Fair bit at higher level i guess) and half length firewall that works on things that are immune to firewall.
At level when you get it it doesn't look too bad, and at higher level it would be interesting against stuff immune to firewall (sure as hell better than tossing a 1 shot spell for twice the SP...). May not be as hot once you get Polar Ray, sure, but...