Bless eats up the concentration slot while bard inspire does not. Bless can vanish easily if the cleric loses conc... Inspire does not.I hear what you're saying in terms of the die being relatively powerful compared to the level you gain it. It's not that I think Inspiration is bad but compared to Bless, Inspiration is only "ok".
Having been both the recipient and giver of both Bless and Inspiration, only Bless has the players arguing who should get it. Hell, I once had the 4 weapon-weilder types basically present their resumes. Inspiration is like "Does anyone need Inspiration?" *silence* "Yeah sure, I guess I could use it". That's because everyone understands that +2.5 to attack rolls and saving throws for the entire combat is better than +3.5 for one attack or save.
And of course, when they get Bless AND Inspiration, players get start to get crazy greedy eyes.
Again, comparing single abilities is complex and is often relatively useless given each comes with the rest of the class.
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