Why would you ever build a casemate tank destroyer at 10 min? It'd be like rushing an SU-85: it'll lose to a circlestrafing Puma and you'll get overrun by infantry because your 135 FU investment does nothing to them and because you rushed it you don't have any AI vehicles.
Because rushing sherman or cromwell or t34 against heavy light vehicles spam of okw is one of the possible "counters". You give jp4 after 2 trucks, you can spam light vehicles all day and if enemy deploys medium tank you go for jp4 because its cheap at only 135 FU and doesnt need shwererHQ. You eliminate any possible weakness of okw and they already have little of them.