I do have various clauses, 'act of god,' 'exclusivity of photographer,' etc. as others have outlined in their answers, and (assuming you're based in the US, YMMV if you're elsewhere) you should have these sorts of clauses too, if nothing else for the following reasons...

Unlike many of the photographers I personally know in the city I live and work, I walk through each and every clause in my contract with the bride and groom, in person, before anyone signs anything. Each bride and groom get's 'the talk' which gives me the opportunity to bring the contract out of the realm of 'legal mumbo-jumbo' for the client, and into the 'how does this apply to me?' realm. Often this gives them the opportunity to handle any problems or prepare a 'Plan B' without me having to do anything else.

For example, I sat down for a contract signing with a couple earlier today and when we talked through what I affectionately call my 'what happens if it is raining at your outdoor wedding' clause. Turns out that they had no Plan B in place, and now they're planning for the 'just in case' scenario. That's a clause of my contract that I will never have to act upon because of a little up-front talk...

I have only had to invoke my 'Uncle Bob the annoying amateur photographer' clause twice in my career, but both times the conversation was me discreetly pulling aside the groom at a lull in the photography explain the situation, and remind him of the clause in the contract. On both occasions the groom then pulled aside a trusted person and that person discreetly took care of the problem. Again, it wasn't a big deal... because we'd already discussed the points of the contract in detail months before, and they were able to see that having 'Uncle Bob' out of the way was benefiting them.

I think that the photographer who just hands a couple a contract, expects them to read it, and completely understand all the implications of what they're signing with no additional detail is just asking for problems when and if a problem arises. Too many photographers I know are simply afraid to talk about the contract and the things that are in it for fear that doing-so may 'taint the deal.' On the contrary, in more than 15 years of professional wedding photography I've never lost a client to going over the contract, never been sued, never had to sue, etc. and I think one of the major reasons for that is simply understanding that even totally reasonable things sound underhanded when put in legalese! By explaining things to clients in plain English I bring everything back into the realm of 'what is reasonable and beneficial for both me and the client,' and reasonable clients don't have problems with my provisions as a result.