Matched bargain trading systems are order driven, rather than quote driven as market maker based systems are. Investors' offers to buy and sell securities are directly matched with each other. This contrasts with a market maker driven system where investors buy and sell to market makers.
The disadvantage of a matched bargain system is that it can not guarantee liquidity. With a market maker system the market makers are required to provide liquidity, so that investors are always able to buy or sell shares. This does not apply to large quantities.
For shares that are in any case very liquid, a market maker system is not needed to ensure liquidity, which is why the London Stock Exchange uses a matched bargain system for the most liquid shares (e.g. FTSE 100 shares), a market maker system for moderately liquid stocks and a matched bargain system for the most illiquid shares (which are so illiquid as to make market making too risky and impractical).