We could do that.
Or - riders could stop half wheeling behind people, and actually hold a straight line which could help things. I understand the argument of "pros are excellent bike handlers because they ride so many miles", but how many of those miles are done in a bunch outside of racing, and if you don't work on bike handling skills, you'll be a danger in the bunch.
When I was watching the coverage the other night, they had a camera mounted on one of the riders bikes, showing a view from inside the bunch. There's guys half wheeling (as in, rather than sitting directly behind a rider, they have part of their front wheel overlapping the rear wheel of the guy in front), weaving from side to side and generally making things unsafe in the bunch. Calls through the bunch about dangers on the road seem to be few to non-existant. I saw Thor Hushovd call out a sign in the middle of the road, and only at the last second were riders moving, and no one was relaying the calls.
Sure, this is a race, but safety should be foremost in a riders mind so they can get to the finish and actually contest for the win.
Additionally, more riders (well, the current count of 198) means that there's more people displaying their sponsors on TV. That's what counts these days. Running a team is quite possibly getting to be very expensive, especially with the cost of fuel and food, so the more air time a sponsor gets, the more inclined they are to keep putting money into cycling. Remember - professional sports are only classed as professional because of the money.