Why
Allowing one account to only vote 7 witnesses helps with decentralization by taking power from bigger accounts. Currently top 20 (+1 backup) witnesses are the ones who decide about hardforks with super majority. Because 1 account is able to vote 30 witnesses, this effectively means that few biggest steem holders could choose the top 20. With 7 votes however, those top accounts would have to split their stakes to 3 accounts to be able to decide the top 20, greatly diminishing their vote power. This would make the chain more secure from takeovers.
Currently a big portion of steem is held by persons or entities that posses great technical skills, which has shown that they prefer voting on other tech understanding people. This can be good in the early stages of block-chain maturity as updates to the block-chain are much more clear and there are still bugs to be found. After this early stage innovation however becomes more important, and as it has appeared to us a few times already, these innovations seem to not come, for the tech smart people oftenly are busy with their other businesses and also do not oftenly make too great innovators. With only 7 votes per account, a 1-3 witnesses voted by smaller holders to top could potentially bring some fresh air to witness meetings and advance innovation so that community driven upgrades to the chain don't just come when the need is most dire.