I'm not exacly sure what you are asking...
First thing- this late- I would check about cancel policies, if you mean to cancel some nights at Marriott. Make sure you don't have a prepaid rate. Also, if you are going to do this - make sure what you want at WL is available.
For me, I usually book everything with a discount. I would not be inclined to trade any room where I had a good discount for a room at rack rates- esp WDW rack rates. If you are happy with the available options though, then sure, you could add a few onsite nights. I have certian had my cahre of successful split stays, so in general I like the idea of them.
Remember though, you will lose time when you transfer hotels. You also will have a gap- between when you check out of one resort- and maybe as late as 4pm. Disney wil stow luggage, but it still takes time to pack and unpack. So the loss of time put into transferring will be greater than the benefit of being at WL for two days.
Usually when my trip gets close...I'm not sure how to say this...but I start to get a mental image of where we are staying. Once that happens, I'm reluctant to make changes.
Also, even if you are staying offsite, you can certainly visit WL (for a break). you can't go to a room, but you can hang out in the lobby and have some relaxation time. I'm also not exactly sure where your particular Marriott is located - but it may well be perfectly reasonable to take a break at your resort. For HS, we usually cover all we want in the park by about 4pm.
