From a (former) wedding professional, I can say a few things about the pro's perspective:
- It's a stressful day for the photographer, the wedding couple, and everyone else with a stake in the day.
- There have probably been a few mishaps so far. The wedding isn't going according to plan (because none do), which might or might not affect the photo process. Did the bride plan on getting the family groupings shot at the altar (and therefore the photographer's been planning on using his off-camera strobes) but the wedding lady announced just a few minutes ago that either this isn't allowed or that there are only 10 minutes available before the next couple comes through? This happens. Is it raining and the bride was dead-set on photos in the garden? Is there turmoil because the maid of honor went into early labor and couldn't attend, but she's the only one that knows everyone that needs to be gathered for the group photos?
- Posed photos don't need to be a miserable experience, but it's nowhere near as fun as drinking and celebrating with all your friends and family. These need to be done quickly, especially when you've got extended family who are just itching to slip off and hit the open bar.
- The bride paid a lot of money to capture the photos she'll have to remember the wedding day. She's likely spent hours thinking about (and communicating with) the paid photographer to make sure the images match her vision. She's chosen the photographer in part because of his portfolio of work. If she's expecting (or he expects to deliver) a great shot of the cake cutting, then he needs to stand in the best location to capture it. It doesn't matter that you staked out that spot 15 minutes prior -- he's paid to be there to get the best shot per the bride's wishes, and it's not a personal insult if he politely asks you to move slightly. It's his job.
- Everything is on a schedule, and everything is invariably late. When the wedding starts 45 minutes late and takes 20 minutes longer than expected, everything else needs to move as quickly as possible. If photos come after the ceremony then the caterers have been keeping the food warm for an hour already, the lighting is no longer as was planned, and it's time to knock the group photos out ASAP. It's great that you want to take a few minutes to capture your own groupings, but everyone is waiting. And the paid photographer will take the blame for lateness.
- Lots of photographers try really hard to create unique group photos. They don't want you slipping in and shooting your version of their grouping, even if they force a smile. This is worse if they happen to sell photos of the groupings but you want to put yours online for free so everyone can print their own.
It's fun for you, but for the paid guy it's a job he takes seriously, as does the bride. The stakes are high for both of them.
If you're going to take photos, try not to interfere a whole lot. If you've never done it, you don't really understand all the issues surrounding what seems to be a simple desire to exercise your own photo outfit on friends/family.
This isn't to say don't have fun, but try to be sensitive to others' needs.
With that said, if I were to carry a camera to a wedding I'd just take 35/50/90mm 135 equivalents and mostly enjoy the day.