Disadvantages
It has been suggested with modern widespread high-bandwidth, low latency connections, Keep-Alive might not be as useful as it once was. The webserver will keep a connection open for a certain number of seconds (By default 15 in Apache), which may hurt performance more than the total performance benefits.
For services where single documents are regularly requested (for example, image hosting websites), Keep-Alive can be massively detrimental to performance due to keeping unnecessary connections open for many seconds after the document was retrieved.
Keep-Alive can cause unexpected behavior if a browser is configured to use a proxy. If a browser establishes a persistent connection to a proxy, it can then send HTTP requests for different hosts over the same connection. If a rudimentary proxy then establishes a persistent connection to a remote server, it may accidentally send it HTTP requests intended for another server.
Read more about this topic: HTTP Persistent Connection