I had been thinking about a post but Marvin Schick beat me to the topic. However, since my perspective is radically different from his, I'll offer it despite being late.
Why do I pray? Because I have to; it's the halakhah. But more than that, I pray because directing my needs to God brings me closer to Him and strengthens my belief. However, the only reason I can pray is that I believe it works. I believe that God hears my prayers and sometimes, when He deems it proper, grants me my desires. It's happened in the past to me personally and to many I know; God has answered our prayers in the affirmative. Can I prove it? No. I have no evidence that these occurences were above statistical possibilities. That notwithstanding, I still believe that God answers prayers. If I did not, my prayers would feel meaningless to me and they certainly would not bring me closer to God. It is only the possibility of being answered that allows me to experience prayer.
R. Joseph B. Soloveitchik wrote (Worship of the Heart, p. 35):
When man is in need and prays, God listens. One of God's attributes is shomea tefillah: "He who listens to prayer." Let us note that Judaism has never promised that God accepts all prayer. The efficacy of prayer is not the central term of inquiry in our philosophy of avodah she-ba-lev. Acceptance of prayer is a hope, a vision, a wish, a petition, but not a principle or a promise. The foundation of prayer is not the conviction of its effectiveness but the belief that through it we approach God intimately and the miraculous community embracing finite man and his Creator is born. The basic function of prayer is not its practical consequences but the metaphysical formation of a fellowship consisting of God and man.However, without the possibility of God answering the prayer, the basic fellowship cannot be formed. It's just me saying the words without meaning them.
In my experience, communal prayer is not answered. Thinking back to every communal prayer gathering I can remember, every single one yielded no positive results. Because of this, I simply do not get excited about communal prayer gatherings. How can I pray when I see no possibility of it being answered? I'm not saying that God can't answer communal prayers. He just doesn't seem to. I'm sure He has in the past, just not any time in recent history. I know, according to the Gemara communal prayers are more effective than those of individuals. But let's face reality. For some reason, we aren't witnessing this.
Here are a few possible reasons why our communal prayers consistently go unanswered:
1. Of all the communal prayer gatherings I can remember, only one strikes me as not being politically motivated. Generally, though, they seem like political rallies with prayer as the excuse. Perhaps God does not approve of our using prayer as a political tool.
2. Maybe we are so spiritually impoverished that we do not deserve the great merit of having a communal prayer answered.
3. Communal priorities might not be optimal. Perhaps we should focus our efforts in other directions. How about a prayer rally for the sake of orphans and widows?
4. Maybe, precisely the opposite, our communal prayers are for issues that are of such importance that God's plans cannot be changed.
I don't know why. Maybe my memory is faulty and we have, on a communal basis, prayed for things that have actually come to pass. But to me, a communal prayer gathering is about many things: communal unity, hinukh, being cool, but not effective prayer.
UPDATE: To clarify, as the comments imply I should, I am referring here to big community prayer gatherings, like when they cordoned off a few blocks in downtown Manhattan about six or seven years ago. Not to regular community prayer or reciting Tehillim for someone sick.