Yes smartbuy is one of the major reasons. And for many people there is feel good factor alsoIs smart buy the only reason everyone wants Infinia? I would rather use SBI cash back and Axis Airtel for 5%-25% cashbacks. Premium cards are for benefits other than cashbacks.
Having premium card is also one benefit, but you already have Marquee. In this case, premium card benefits of Infinia are not going to stack above Marquee, they are mostly same. (Well the usable ones... not talking about Infinia's ITC 2+1 etc.. which are non usable mostly)
Also when you say "rather use SBI cashback and Axis Airtel" - Yes you should have both of them irrespective of whether you have Infinia or not. Their unique benefits cannot be replaced by Infinia benefits under some conditions