Consider this half an answer or an answer to half your question :)

Most cameras on the market use custom batteries. This allows the camera manufacturer to provide a precise and reliable experience for the user. The downside is increased cost and inconvenience since options are limited.

AA batteries are great and Pentax still makes DSLRs that use AAs and can reach battery life above the majority of similar entry-level models. However the latest models, both the K-r and the soon-to-be-available K-30 support BOTH types of batteries. The reason is there was a lot of complaint with people getting very poor performance from AAs, far from the numbers provided by Pentax. With Lithium AAs on a K-r, you can get over 1500 shots!

The real problem is that most people who do not pay for proprietary batteries do not even pay for quality AA batteries! A good set of 4 AAs costs about $15-20 USD, depending on where you live. PowerEx and top-of-the-line Energizer or Sanyo as well as Imedion and Sanyo Eneloops fall in that category. The last two are less powerful but low-self-discharge so keep 90% of the power after 1 year without use while the first two empty themselves in 1-2 months. If your batteries are Alkaline, have a cat on them or say GP, consider them garbage.

Honestly I really like AAs and I was annoyed when I upgraded to a camera with proprietary battery particularly since I have been away from electricity for weeks at a time and a Solar AA charger purchased for about $25 served me well. For Li-Ion batteries a solar charger is not attractive because I would need one for each type of proprietary battery.

There are battery grips for other cameras which support AAs but I have not seen one for the D3100 other than the link you provided.

PS: I know how poor AA performance has been for those who skimped because I got hundreds of messages from people wondering why they got such low battery-life.