As reflected in the two different opinions, I think feelings and tipping practices vary depending on the origin of the particular traveler.
If you're from Europe where tipping is not considered mandatory, and when done is a small amount, (say, like rounding up to the next Euro, or the change from paying the bill); then I'm sure it would seem crazy to tip at an A.I. where you feel you've already paid for everything.
If you hail from the U.S., where tipping is considered an obligation, and an integral part of a server's income; then I'm sure it would seem natural to tip, (despite it being an "all inclusive").
I'm from the U.S. and feel numerous small tips are appropriate and genuinely appreciated in the D.R. (I visited an A.I. in Riviera Maya, Mexico, and it didn't seem that the tips were as appreciated quite as much.) In general, I tip a dollar or two per day to the chambermaid, waiters in the buffets get a couple of dollars, in the a la carte restaurants a few dollars to the primary server and a few to the maitre d, bartenders get at least a buck for each service (one or two drinks), if it's a bunch of drinks they get a couple of dollars (I like to keep the bartenders VERY happy, lol). Occasionally I'll tip other staff, (front desk, room service, etc.) for out of the ordinary services. These people work long hard hours for very little pay, and I think they earn the tips they get.
Does this get you better service? Maybe, but it's not a guarantee. Service will be good regardless. These are highly prized jobs, and people want to keep them. They are dedicated to helping you enjoy your visit. Actually though, I feel it's at least as much, maybe more, important to treat the hotel staff with respect. A friendly smile, and a "hola" goes a long, long way. Sure, the money "talks", but it's not the bottom line. If you tip a couple of dollars and treat people as your new amigos, and then the next guy tips, (maybe even more), but treats people as servants, slaves, or worse; guess who gets the better service?
If you speak any Spanish, even a little, it's fun to use it. Many of the staff will try to help you learn a few words, and actually pride themselves on their language skills. (They need to have at least a working knowledge of many languages.) For me, being friendly with, and getting the chance to learn a little about people in a different culture is a highlight of international travel.
Over a week's stay, my family will go through two or three hundred dollars in various tips. That could actually be more since I never know for sure what my son spends from his own funds. It sounds like a lot, but compared to $6K US and up for a family of 4 adults at a resort for a week, it's not that much. Am I a Gringo tourist sucker? I don't think so. Of course I'm also the guy at the airport that lets the porters handle my luggage, even though I'm quite capable of it and it has wheels anyway. They're just trying to earn a living and that's better than having people beg in the streets.
On my first trip I was really stressed about the whole tipping thing... didn't want to over tip and certainly didn't want to look like a cheap skate... I was just unsure about the whole thing. In the end, it doesn't matter very much. Tip what you feel comfortable with, don't sweat it, and enjoy your visit. After all, it's YOUR vacation!