Su76 and T70 were both a light vehicle option, depending on tech choices. You'd choose one or the other with tech. Now that both come from the same building, there's no reason to get a su76 ever.
I'm not entirely sure you know what you're talking about. You're basically saying something like, "Now that Combat Engineers and Conscripts are in the same building, there is no reason to make Conscripts." It makes no sense! You need Engineers for flamers, building wire, mines, sweeping, repairing, and building. You need Conscripts for fighting and being versatile core infantry.
Likewise, T-70 is a light tank that specializes in anti-infantry. You get it if you want some major harass and to try to bleed the enemy or wipe his squads, as well as transition into a late game map hack. You get the su-76 if you want a light TD to fend off your enemy's tanks and have additional barrage options for his infantry. You're not going to be running around with an SU-76 to try to wipe squads. You're not going to sit a t-70 in the back to snipe medium tanks.
Situations like Soviets needing a light harassment vehicle AND protecting it with a light tank destroyer can come up and it does very often in this game. I just can't wrap my head around why you would think that, just because you have a t-70, there is no reason to increase your AT capabilities, or if you have an SU-76, you won't need a harassment / recon vehicle.