Do we need to go as far as turret bustle? Arjun has this drawback, but it may not be a major obstacle in a tank vs tank battle within a smaller safe maneuvering angle, unless of course if it is compared to M1A2s or T-90s design.
It depends, for Americans it was somehwat nececity to also give enhanced protection over turret bustle.
Mainly because tank was developed over a priority to give crew higher survivability than any other tank can offer, this is why in turret bustle there are two separated ammunition magazines, these are not only separated from eachother but also separated from crew and it was far more easier to install such magazines with blow off panels in turret bustle. Because turret bustle is exposed much more than hull, it was nececity to add protection there, if not then any hit could end with loss of allmost all ammunition stored in tank.
This is a tradeoff, higher crew and vehicle survivability, but also bigger weight. On the other hand west european tanks have at least some part of ammunition in hull (well in case of Challenger 1, Challenger 2 and Ariete all or allmost all ammunition is in hull), thanks to this turret ammunition magazines can be smaller, or there is no turret magazines at all, making vehicle overall lighter, however tradeoff for this is low crew survivability in case of armor perforation and possible ammunition cook off.
Arjun was designed in very similiar way as western european tanks, however, as it was pointed out, due to lack of composite armor protection over turrets sides when it comes to at least crew compartment, made vehicle highly vurnable in tank vs tank combat withint vehicle safe manouvering angles.
What is more important, composite armor over turret sides is a very usefull addition for assymetric warfare and urban operations. It is a very big mistake to see city as a deathtrap for tank, no it isn't if vehicles are used properly with cooperation to infantry and other assets. Also what is more important, vehicles need to be prepared for such operations. In case of Arjun this can mean deinstallation of storage bins over non composite armored turret sides, and installing there some sort of modules or ERA. But it is easier if such armor is allready integrated in to vehicles structure and not developed when need suddenly become obvious for everyone.
Let's make a short analize of all urban addon armors for NATO tanks. Americans use ERA (currently a multilayer ERA to be precise), mostly because it's highly effective yet light and easy replaceable solution, Germans use Composite armor modules, these are probably heavier than ERA or are some sort of low density composites optimised only against HEAT and EFP's to maintain low weight. British use ERA + Composite mix, it's highly effective yet incredibly bulky and heavy bringing Challenger 2 max combat weight to 74 tons, such weight is rather unacceptable. France seems to used ERA on Leclerc AZUR demonstrator. Even C1 Ariete was used in Iraq with composite armor modules protecting hull and turret sides over crew compartment, these were similiar to MEXAS armor modules seen on Canadian Leopard C2 (Leopard 1 variant).
But besides Ariete all these tanks have at least crew compartment in turret, protected from sides by composite armor, it is a good thing, because if there is no need turret might be using only it's basic protection, thus some weight is saved, and if there is need, additional protection can allways be installed.
Please remember that additional weight is closely connected to vehicle wear and tear, as it's mechanical components. So different solutions need to be analized. But as India mostly seen use of tank in classic conflicts and tank vs tank engagements, after detailed analize of tanks design, it is safe to say that T-90S in at least armor protection, might be better than Arjun (an definetly not worse), and it is highly probable that Pakistani modified T-80UD's (Object 478BE) are better or at least not worse in terms of armor protection. And highly exposed, not protected weak zones in Arjun turret structure might be very dangerous.