I think there's a misunderstanding here. LMDE 2 does indeed have some newer packages than Mint 17.x (though not than Mint 18), but that doesn't mean it has the absolute latest packages:
* Mint, and LMDE, doesn't have the goal of having the latest packages. Our goal is stability and user-friendliness, both of which are difficult to achieve if you constantly integrate newer packages (which can, and often do, break things).
* Mint, and LMDE, does indeed have the latest packages for some programmes, such as Firefox, in which case it would be a security risk to use older versions.
* LMDE 2 is based on Debian 8 Stable ("Jessie"). We get most of our packages from Jessie, and the Debian team is very particular about releasing stable, well-tested software. If you consider this a bad thing, then please at least don't blame LMDE. We are dependent on Debian and their choices.
(Please note that I consider Debian's stability to be a great thing! I don't have time these days to debug a broken OS, and I suspect most Mint users don't either.)
* As a wise man once said, "A stable kernel does not make a stable OS." Just because kernel 4.8.6 is stable does not mean that a Linux distro which utilises it will be stable.
(Actually, I just came up with that quote.)
* As Yoda said, "Outdated, a package is not, when newest, it is not." LMDE, and its packages, are not outdated and won't be until they are no longer supported by the Mint team and the Debian Security team (around 2019 if I remember correctly). Yes, there are newer packages (with new bugs). But we are still being supported and so we're not outdated.
(Yes, I just came up with that quote too.)
* Lastly, you can get some newer packages on LMDE through Debian Backports.
If you want the absolute latest packages, then I recommend Arch, or (the more newbie-friendly) Manjaro. They're both outstanding distros, but be warned that you are much more likely to break something. Be sure to make regular backups!
EDIT: or Debian Sid if you want the Debian package system.