Multi-member districts cheat the voters out of clear choices in their elections, are open to gamesmanship and are more easily gerrymandered than single-member districts.
Multi-member districts are districts with multiple members of the legislature serving the same geographic area. These districts are far larger than they should otherwise be, and have a number of slots (generally 2 or 3) to fill with the top vote-getters.
There are multiple problems with multi-member districts (MMD). First, MMDs insulate incumbent politicians from losing elections by allowing members to survive their election in as low as third-place. This means that incumbent politicians don't have to win elections, they just have to avoid losing them.
It also makes it nearly impossible to campaign against an incumbent. Since the incumbent only has to come in third (in three-member districts), they will survive a challenge if there are only three candidates. Any attempt to unseat him in a competitive race would have to be an exclusively negative campaign to get people to not vote for that candidate, giving that target of the campaign tons of free name recognition and the chance to cast himself as the underdog. Whereas in a single-member district, such a campaign could focus on promoting the alternative, rather than simply butchering the incumbent. In a MMD, such a strategy could result in both candidates achieving office.
In addition to that, MMDs lend themselves more easily to gerrymandering and gamesmanship. The more space and population in each district, the more opportunities there are to rig districts in favor of one party. To make the point clear, imagine if there were only 7 MMDs in Maryland, aligned to the Congressional map. The only Republicans would come from the Eastern Shore, and there would be almost no opportunity to expand the party beyond MD-1, ensuring permanent Democratic control.
On top of the ease of gerrymandering districts, MMDs can also be split in Maryland. So, in the case of a strong Republican district with a Democratic enclave, Annapolis Democrats can split off the enclave and net themselves an extra vote. Likewise, in a competitive MMD, Democrats can pack enough of the Republican voters into one sub-district, spin it off, and make two seats secure for themselves. What's more, this process is optional and expressly open to gamesmanship. This allows Democrats to maximize their legislative control without having the voters that would otherwise be required.
Even unpopular incumbents can continuously win re-election because the size of MMDs discourages challengers from running. The larger the district, the greater protection the natural advantages of incumbency provide.
Larger districts enhance the effectiveness of spending disparities and media coverage. In a small district, a large advertising purchase or media campaign can be partially overcome simply by through direct contact - knocking on doors, waving signs, small public events, etc. The smaller the district is, the less effective advertising or media are. But the reverse is also true - large districts are much more difficult to campaign in, because the costs are greater. Again, compare the effort and money needed to run in a Congressional race with the effort needed to win a *current* statehouse seat.
MMDs also insulate incumbents from accountability. Whereas the representative of a small district is likely to be well-known in the community and easily reachable, the representative of a larger district will find it easier to ignore or evade difficult problems or challenging voters. They may hide behind staff, be unavailable, or simply pass the matter off to another delegate in the district. This also diffuses accountability - which of the three delegates do you blame for not addressing an issue?
MMDs are perfect for avoiding responsibility and cheating the voters out of a fair choice. That is why they must go.