hmmmmm i've never been to a wedding where cake wasnt served for dessert altho i've been to one where they served a different cake form the back that looked nothing like the wedding cake they cut)... i dont think it's a rule that you must offer wedding cake! some couples opt for cupcakes, or pies, or gelato, some couples offer a dessert buffet, ice cream/sundae buffet bar with all the toppings.
what kind of dessert are you offering? it may be confusing to the guests at first, but it's your wedding - do what you feel is best kwim?? i'd say go for it if that's what you and spencer want :)
the last time i was a bm - they had wedding cake - but then also offered a dessert buffet so if the guests didnt want cake they were able to choose their own dessert.
if you have a sheetcake - you may be charged a cake cutting fee if it's form an outside bakery...
also - i've been to a wedding where my cousin did this - and some of the guests grumbled about not getting the "same" cake (ie i overheard my aunt complaining - "what were they too cheap to get us the same cake?")