I think I really don't get how staying on 1 pool that has a large share can cause problems.

I'll ask that someone who's more knowledgeable than me tell me the problems in an easy to understand way.I've read th ecomments here but some it seems to be hearsay (I'm not sure if it's true or just an unfounded fear that some ppl have).

My questions are:
1.How does mining on 1 pool that has 50% or more share cause problems for hte BTC currency if any?
2.If the service provided by the pool with the majority hares is very good,why should people switch to pools that have a smaller share possibly due to a possible lower standard of service for its members?
3.I don't know the share for my pool bitcoins.lc.If that's too large and it's causing problems then I won't mind moving pools.
4.If I join a pool with very few members,will I get a bigger slice of the pie with my current hash power of 400MHash/s or do I always get the amount based on my hashpower,size of pool,size of payout minus fees?

because I typically get about 0.25BTC in 14-17 hrs on bitcoins.lc so If I joined a smaller pool will I get more for the same hashpower because of the far fewer members?

Thank you.