You are assuming because the mechanism is the same that the effects will be as well - it's like saying "we have this black box service we sort of have an idea of what it does and if we send it X/Y/Z we know what happens, it should work the same with W too - I've been in that scenario often enough to know that's an assumption that I'd rather see the data for.
I already saw that people in my peer group reported short term side effects like headache after getting vaccination, have taken days off from work, family members complained about dizziness the day after.
It's relatively easy to do these studies along with long COVID studies, should be insignificant even compares to money that will be spent on vaccination campaigns - so why skip out on valuable data.