There's an interesting discussion on the Elitist Jerks resto-druid forum on the question of how much value should be placed on proc-based throughput. This is highly relevant because the Mandala of Stirring Patterns is one of the best pre-raid trinkets available from the point of view of sustainable healing per second. However, a lot of its throughput comes from an int proc that only has 20% uptime.
Note I am writing here only about the throughput improvements. Everyone agrees that the regeneration benefits of a proc are at least as good as a static improvement that has the same average benefit. As long as you finish the fight with at least one mana left, you don't care whether your mana descends at a steady rate or that it goes up and down in spurts as your procs come and go.
To evaluate this question, one premise I assume is that we are talking about progression content. It is a separate question to consider content that is on farm, where your raid reliably beats it and you want to beat it more reliably and more quickly. For such content, it doesn't matter that much what you do, much like it doesn't matter how you level, so I find it much more interesting to talk about how to address progression content.
For progression fights, it's highly unrealistic that players will be at max health throughout the fight. What's more likely to happen is that players hover around 50-75%, and you race to get them above 75% before the next wave of incoming damage happens. The cycle is that health bars drop after big damage, they float slowly up as healers heal like mad, and then they go down again at the next wave of damage.
If your healing is sufficient for the content and for the skill of the raid members at dodging fire, then the peaks and valleys of health in the cycle will be about the same. When I see my guild lose in such raid encounters, it's due to either unhealably severe spike damage, or it's due to players standing in fire. The former I can't do anything about. For the latter, I theoretically can help with, but when I fail to do so, what I see is the health bars going down fairly gradually.
For the content I'm familiar with, which is about half the Cataclysm raid bosses, the length of the cycle of hurt is on the order of thirty seconds. For example, on Argolath, a meteor slash comes in around 10-15 seconds, but it hits a different half of the raid each time. Thus you have 20-30 seconds after a player is hit by one meteor slash to heal them back up for the next one. Furthermore, any one wave of hurt tends to take off something like 50% of everyone's health bars. It's pretty rough, but it's not like many Wrath fights where everyone would suddenly lose 75-90% of their health.
How does proc-based versus static throughput compare in this environment?
It seems to me they are actually pretty close. First of all no proc is going to be wasted. Since players aren't at full health in the steady state, there will always be plenty of healing to use the proc on. Second, if you don't get a proc at the ideal time, it should just mean that players hover at a lower health for a little while. You will be able to pick them up just fine when the proc eventually comes around.
This is a minority view on the thread I linked. The majority view is that it's better to have static int instead of an int proc even if the average int goes down by 10-20%. The main arguments are just what I addressed above: they argue that many of the procs will be wasted, and that the procs might not be there when you need healing the most. In short, the proc won't save lives. I just don't see it. Saving a life only requires an emergency heal of 10-20% of a player's health, and for healing on that magnitude it doesn't matter what your immediate throughput is. What's more important is whether you can push the raid back up high enough to survive the next spike, and for that procs are fine.
Perhaps things are different in heroic mode, and there is more a need for emergency, unpredictable heals. I wouldn't know. At the rate my guild is going, I'm not sure we'll be doing much heroic content before the next tier comes out.
As well, I'll certainly agree things are different in a lot of ways once you master a tier and have everything on farm. You'd rather avoid proc-based gear for on-farm content. Additionally, sustainable throughput is much less interesting, and in particular haste is a lot more valuable.