Because different people have different tastes. So having a little bit of everything to please everyone is always nice. Before I can make solars, I burn coal coke, use creosote oil in the semifluid generator and place lava cans in the thermal generator. Why I don't still do that is because I can use the coal coke in other burning machines, use the creosote oil for RailCraft and use the lava for centrifuging instead.Well of course you can use solar for everything, and since you do, why have other ways to produce energy (like gregtech) if using solar is enough?
Just gonna say it again. I'm a hoarder. So I try to save as much as I possibly can (of course I still give things to people if they need them. But that's beside the point.)
Also, I play with gregtech on because of the nice machines and game balancing (Because IC2 was SO easy to blast through into OP mode normally. Almost as bad as EE2 if you ask me.)