Just read a review on the Madden 15 Xbox One bundle. In the comments of the review, someone named WeaponX claims he's able to play Forza 5 (he has the disc version) without the disc in the drive. Below is the relevant part of the comment and link to the thread:

"Now, here's the HUGE plus, I'm noticing Xbox's grand scheme finally coming into play. They had talked about using the discs as key codes from the start, meaning once the game installed you could put it away and load from the console, this is actually becoming a reality! I was playing around the other night and hit Forza (without the disc being in) and low and behold it loaded and played (in full) without the disc even being in, this is the GREATEST thing and I'm hoping it is a sign of things to come!"

http://www.amazon.com/review/R3BK4L3EQ6RP2V/ref=cm_cd_notf_message?ie=UTF8&cdForum=Fx1FWUYS5Q8P80J&cdPage=2&cdThread=TxNHV9K49S2JRU#Mx32048A9HQMJA6

Maybe because it's free this weekend, so it thought you owned the digital version, since you had to install the game anyway. Still, pretty awesome.

I will check again, but it wouldn't let me do this with Dead Rising 3.... will be right back.

Edit: Nope, didn't work. It asked me to put the disc in.

Forza is free this weekend. The same thing happens with EA Access, you don't have to re-download digital versions of installed games.

Yeah this is due to the fact its free for everyone that has gold this weekend once that promo is over he once again has to put in the disc to play it.

The problem is that IF they did this, there is nothing to prevent you from loaning or selling the original disc, nor to restrict the next person from doing the same thing. People would quickly exploit this and either rent the game instead of buying or just trade in the disc after it was installed.

If you want the convenience of not having to put the discs in the drive when you want to play, but digital...but then you can't trade/sell/loan the game to anyone else.

Personally, I have bought ALL of my software this gen digitally. I tire of losing discs, scratching them, or loaning them out to people who never return them. It's worth it to me to just buy it and never have to worry about it again. Then again, I was never much of a fan of buying a game for $60 and trading it in to gamestop for $10...so, there's that...

The only reason it works with Forza is because its free for anyone that has Gold this weekend (Friday through Sunday).

Yep, that is the reason why.  Would have prefered if they would have added more DLC cars instead, but maybe there is still some coming out later this month if not the demo for Horizon 2 will be out in less then 2 weeks.

In the case of Forza I thought you needed at least go to the store and activate the free weekend "voucher".

Interesting it is automatizated. Does it mean that if a game comes as "Game with Gold" you don´t need ever the disc?

Actually I was doing this for awhile with NBA2K. It finally just recently asked me for the disk. I'm guessing an update got me. Don't know how or why. Didn't care because me and my nephew were playing at the same time lol