Current layer 2 link aggregation is effective for lots of connections with different source/destination pairs because the switches have limited resources and require simple schemes. It works fine for public servers and transit aggregation, but it's very bad at local aggregation to speed up limited connections like trying to bond multiple 1gig ports between san/nas and a server for example.
Exactly. With Multipath TCP, a single TCP connection can benefit from multiple interfaces, which is not the case with bonding techniques. See http://multipath-tcp.org/pmwiki.php?n=Main.50Gbps for measurements showing Multipath TCP aggregating 6 10 Gbps interfaces.
