Whilst the traditional notion of 'all butter' baking sounds like the indulgent, quality way to go, it can often result in tough, solid cakes.
When baking light sponge cakes (and note this is all about cakey baking not pastry baking) I find that margarine or those 'It's not really butter but we tricked you (haha!) into thinking it really is butter' products are the best thing to use.
This is because, other than when it's being mixed or fresh out of the oven, butter is solid. Using straight butter in a cake that's going to be stored in a cool place for any length of time will result in a pretty solid, heavy cake.
That being said, you don't necessarily need to double up on your dairy products every time you shop. If you are anything like me and get a bit freaked out when product ingredient lists stuff that should only be kept in a lab somewhere and not in my pantry, you'll be better off making your own.
Simply mix together 3 parts butter with 1 part oil (use a flavourless oil like vegetable, sunflower or canola) to make your own baking butter. You will still have the lovely rich flavour of the butter but have a lighter consistency when cool, and without a whole host of weird stabilisers, flavourings and chemicals.
I tend only to mix up a batch to satisfy which ever recipe i'm about to bake, but any left over will keep in the fridge until the use-by-date of the butter you've used.