This is true, usually as you progress some spells or skills are getting obsolete further down the road.
There is an other apsect to it, too. Some time ago I watched an interview with a Magic the Gathering designer (don´t have the link at hand unfurtunatly) who argued that weak cards are getting designed in purpose. According to him it makes discovering strong cards much more rewarding. Of course, comparing a TCG and with a CRPG is a bit pointless, but maybe this was in part a factor for some less usefull spells/skills.
Blood Rain is situational, true. It´s decent if you have a summoner/necro or build for it to synergies otherwise.
Just for the sake of mentioning it, I tried a "I heal you to death" party setup just for fun and with this group, Blood Rain became a core spell. Not particular strong, but manageable, even on tactitian.
The bummer at least for me was to find the least usefull spells (or those where it´s hard to fit them in any build) by combining spell schools.
