It kinda depends on the area of the forum that you're replying (for instance, if you're in Forum Lounge, where posts don't count, and just posting in some Forum Game, or in Roleplay World, brohoofs might not be that important to you), but I'd say in most areas of the forum that the brohoof system does indeed serve as one of the incentives for higher quality posts for members, especially new members attempting to establish a good reputation. I remember that when I first joined, high quality posts were always at the forefront of my mind since I'd be more likely to get brohoofs, and building a high brohoofs-count is still one of the more visible ways to reflect a good, well established reputation on the forums.
Of course there are plenty of other incentives as well for posting higher quality posts. Anypony who reads your posts will see that you have something worthwhile to say, and thus be more inclined to pay attention to it, as well as maybe even friend you down the road. The 100 character minimum in most threads is also another good incentive for high quality posts, and has certainly proven effective at assuring them, probably even far more than the brohoof system ever did, since being implemented. But still, as I said, brohoofs are still, for many members, a big incentive for high quality posts for sure, and I think that the brohoof system has definitely encouraged quite effectively such positive behavior from members.
EDIT: Zoop did make a good point on the first page about how, sometimes, posts lacking content, or even posted by certain members such as section mods, end up getting the most brohoofs, and while I don't begrudge this fact, it's certainly an accurate statement to be sure. Heck, to date, my most popular post was a Princess Trollestia joke that I made in the middle of an episode this season that took about two seconds to post! I don't know myself how this could possibly be changed (although let me reiterate again that I myself don't begrudge anypony, mod or otherwise, who get a ridiculous amount of brohoofs from site announcements or posts lacking in content), but it's interesting that the mod team is looking into a way to reform this at all. I guess the only thing I can think of would be if, say, for site announcements, the mods added a "recognition" button next to the brohoof button for members to press, since I think that in most instances with site announcements getting brohoofed, it's really just the members' way of saying that we recognize the change that's being announced and approve. Still, as I said, it's not that big of a deal guys, so don't worry that some posts get more brohoofs than others even though they may be lacking in content. It's all good guys, it's all good.