In addition to KRyan's excelletn 3.5 specific answer, I think this question is answerable as a question about experience point systems in general and what they are supposed to do from a game standpoint. So in that light, I will attempt to answer the question. I have had extensive practice fiddling with experience systems, as I don't think I have ever used the printed method in a campaign I have run. So hopefully all of my experimentation can lend some insight.
Giving out XP for things that aren't specifically addressed in the rules (like attendance and role playing) is only a problem when your players don't find equity in how these rewards are given. Your specific example of attendance, therefore, is problematic because one player is at a distinct advantage.
Skinner Box-ing Match
At the heart of it, every character growth system is the game's built-in way to tell players what they should be incentivized to do. They are little reward packets that give players almost instantaneous feedback that what they just did works, is a good thing to be doing, and will provide them with benefits in the future. It's why XP is one of the most powerful motivation tools at the GM's disposal. In D&D, playing with Rules as Written, characters will gain the most XP from "defeating" challenges, which mostly means killing things. So they are taught that killing things is a good and healthy activity to participate in, and will ensure that they try to become better at killing things, and will kill more things in the future. It's a positive-feedback loop.
So what does this teach us? That rewarding XP outside of how the game system explicitly instructs us to is a way to teach our players what actions and activities are and are not acceptable and encouraged in our game. So there is nothing inherently wrong with offering XP for a range of given activities, including roleplaying and attendance. The problem arises when the players start feeling cheated due to perceived imbalance in how these rewards are given.
Equity, not Equality
Human beings really, really like feeling as though their situation is equitable to others- that is, we don't really want to be the same as everyone else, but we want to be afforded the same opportunities as everyone else. We don't want to feel as though the system is unfairly giving an advantage to someone else, nor do we want to feel as though a system is limiting our ability to excel inside of it. This is why you are experiencing resistance from your players about these additional XP rewards.
They all have the same tools to achieve inside the game system. Everyone has the ability to make a character that is really good at killing things or overcoming obstacles. So even if someone is better at it than someone else, each player had the same starting point, so no feeling of inequity is garnered. However, in your other two examples, people are starting on an uneven playing field.
Attendance- some people have busier lives. Some players sometimes can't make the commute. And they're right about the host- (s)he is given an advantage because without them, there is no session, so they are never in danger of missing experience. If there were rotating hosts, or another way of making up this disparity, I believe the resistance would drop significantly to this point, because then most people are starting from a roughly even place in receiving this reward.
Roleplaying- some people are shy. Some people don't feel as though they are good at acting or coming up with "in-character" things to say on the spot. Some people don't care about the role-playing aspects to the game (which is okay). But I think there is a very easy fix to this that solves the problem while still encouraging role playing.
Our playing group has a "spotlight moment" system. At the GM's discretion (or when the player thinks that their character is having a really cool dramatic moment), the GM pauses the game to have a "spotlight" shined on a particular character. That player than gets to have a monologue about what is going through their character's head at the time. It can be anything from a review of the situation from the character's perspective to a rousing speech given to bolster the army before a battle. It shouldn't last more than a couple of minutes, max. After the monologue, the GM rewards the player with a standard packet of XP, and then play resumes.
This does two things- one, it gets the players into the heads of their characters more and two, it standardizes the XP so that each player has the same opportunity to gain the reward.
We generally try to have these spotlights on each character once a session, but if you just keep track to make sure no one gets two without everyone getting one, you should be fine.
Also be mindful that you should be lenient with the XP you give in this fashion. Only if someone is clearly not making the attempt should you withhold the reward- we are encouraging behavior that some may not practice, after all.
Alternate conditions for earning rewards are a great tool the gm can use to shape his game, but must be delivered in a method that the players view as equitable, and must be used to incentivize actions that increase the amount of fun for everyone involved.