Why would they waste time and money (more than usual that is) on pair of guns which have never been used on a couple of ships which will be in service for couple of decades more?
Because they already spent money developing the guns in a joint venture with the Army, and when the new build Destroyers are ready they will likely be fitted with these guns as will the new cruisers they design to operate with their carriers when the upgraded Kirovs and Slavas are retired.
It is a chance to get the gun on a ship and tested and fully operational so that when the new destroyers are ready to be built they will have all the bugs and kinks ironed out and also have a powerful gun system at sea ready to use much earlier than if they leave it till their first brand new destroyer hits the water.
Having naval guns able to use this ammo means larger production volumes so it will be cheaper for both branches and offer capabilities than the current 130mm guns on their frigates can't match. In comparison the 130mm guns reach about 25km with a 34kg projectile... even just current 43kg 152mm shells reach 70km or so with GLONASS guidance.
Navy R&D money developing new anti aircraft and other shells will make the Army ammo cheaper and vice versa...
Hell with a 170km range with a reduced HE payload they could use the damn thing to direct depth charge attacks on submarines 150km away from the ship and land 90-120 charges a minute if they want to...
Lord knows they have screwed up enough on overhaul of first one and they are supposed to fiddle some more with something that is glorified ballast at this point in time?
Yeah, using upgrades of existing types to test new stuff is silly... much smaller to make brand new types where everything is brand new and untested so when something goes wrong you don't know what the real cause is so you have 1,000 different problems with the design all at once and you don't know if it is the system itself at fault or a component in another system it should be interacting with... so you just blame something else that is not working for your part to not work at the moment... ie Ford class CVN, Zumwalt Destroyer class, LCS frigate wannabe...
Upgrades of the Kirovs and Slavas are going to be tricky because it has not been done before so there is no rule book, but it is a good chance to test new technology that is going to go on their new build replacements... otherwise they could keep using the Granits instead of the UKSK launchers and save some money...