I've probably had my fill on MK8 and I can always put in my Wii U version when I fancy a quick go. For me the only factor for MK8DX is the portability. Battlemode might be neat to try (that new cops and robbers mode could be fun), but I've preferred the racing for a long time now (since the SNES days time).
Would it be nice if Nintendo gave me a discount. Sure. But when I got MK8 to begin with they gave me a free game, which is turn gave me £5 for the eShop thanks to their digital games promotion, which I could use to get the DLC for even less. The tshirt and keyring I got with the blue shell LE were also nifty. So I basically I got my money's worth and loyalty discounts with the original in some ways.
Basically, if there's a rare promotion to get this for £30 I'll probably double dip, but I'm happy to pass.