Impulse wasn't the first Gundam to be formed by multiple parts. There was also V Gundam and ZZ Gundam. I do agree it's kinda silly to have all the parts fly out of the minerva separately instead of merging to form impulse Gundam from within the minerva. But they ability to switch parts is kinda cool, remember the time when the legs were destroyed Shinn can just ask for another set for replacement.