Generally speaking, you can't sub a liquid fat for a solid fat. Liquid doesn't hold air, first, but liquid fat will always be liquid - it will never solidify like butter or shortening. I have tried to sub liquid oil for solid fat in a hi-ratio cake and I got a spongy, wet, oily mess.
The exception is if you are doing a chiffon cake, which calls for liquid oil and no solid fat. But it relies on meringue to hold air and cream of tartar to strengthen the proteins to keep the structure. The ratios are different as is the mixing method of a hi-ratio cake.
Shortening is used in white hi-ratio cakes for a few reasons - shortening is white, butter is yellow. If you want a white-white cake, you use shortening. Shortening also has natural and chemical emulsifiers that help bring your batter together, making for a more perfect crumb. Again, generally speaking, shortening actually lightens cake, not makes it more dense, much like it lightens pie dough, cookies and pastry.
You can sub out shortening for butter easily if you don't want to use shortening.