I maybe a bit biased, as I have Nids aswell, but I think they are just being arses.
A Tyranid army should be a swarm, thats its nature. I think making gaunts as cheap as possible to make this possible, is playing by the fluff of the army.
With the rule "shoot the big ones" (which should be shoot the genestealers and then Warriors, etc
) we need distracting units.Spine gaunt are not as good as termagaunts. The flesh boarer is S4, basic spine fists are S3. Thats a large difference.
I think your opponents need to just formulate a plan on handling tyranid armies. Playing against Nids requires a balanced force. Cheesed out armies can struggle against them. Armies where they have taken a lot of heavy weapons may knock out the big 'uns, but they will be overwhelmed by the numbers that they wont be able to handle. Armies that have a million heavy bolters (for example) will be wasted as the big uns arrive to stomp them into the ground. Opponents need a combination of these weapons to succeed. A balanced force. (Tau do well).
Look at their armies. Are they specifically aimed at a certain role? I'm not the only Nid player by me. And whilst I dont use spine gaunts often (as I have put the toxin sacs etc on them) another player uses them. I have seen him lose as often as win. OK maybe when the opponents know they are gonna face him they prepare. (whirlwinds backed up by dev squads, with lots of tac squads and bolters, that sort of thing). But balanced forces should deal with him OK anyway.
If they are still complaining, level with them, and use just one unit. I dont see why you should alter your force though. If they take a lot of star cannons or heavy bolters against you, complain back.
