I completely understand where you're coming from here, and although I am willing to increase the number of cores each week to something like 4/5, I don't think that building creatively against a common core is necessarily a bad thing. Sure, you have to cteam the other core, but the competition drives creativity by participants adapting to what is being done to counter their core. You could think outside the box and, just off the top of my head, run Sunny Day on your Heatran to stop Pelipper coming freely in and setting the rain. In a way, this situation simulates acceleration metagame trends - how one thing reacts and adapts to another. I know this probably sounds a bit over-imaginative, but overall the competition is as much about inventive teambuilding as battling skill. I encourage you to not stick within the 'traditional' confines of a rain team, but have some fun and experiment - that's surely one of my favourite things about playing Pokémon. But yeah, there will be more cores next week :p
Edit: greninja'd by Frat Dude, I appreciate the feedback and I'll try to change that next week