No you're doing it correctly. In an assault it's FF or CC, no ranged weapons.
Your question is really, what do I do with 15cm range weapons (incidentally plenty of weapons are 15cm such as the multi-melta)
First, you might want to hold an objective with them after you've taken it. An overwatch shooting attack would be handy, no?

Second, assaults are do or die. Its easy to pull them with a double up near an enemy and shoot from 15cm, laying BMs and perhaps killing some guys. Then you retain with a bigger formation to assault, like a mech company or a warhound and those same stormtroopers can add support to the assault.
Third, 8 dice at 5+ equally good at killing infantry as vehicles mounted up in a speedy skimmer transport isn't a shabby deal. You don't see them quite as much as there's stuff more killy for the points available in Guard armies. That still doesn't make them useless.
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