Still trying to sort out the different advice from crawlspace drainage vendors. The least expensive fellow insists a single 1/3 HP pump pumps out plenty of volume, and that only a single pump is needed (w/battery backup) because they're so reliable. He also said he stopped using sock filters because he found the socks themselves were much more likely to clog up than the perf pipe. The other, far more expensive operations, are pushing for a 3-pump system and insist a sock is mandatory. Hard to know who to believe. Thanks.
If you have any fear of high water table of flooding you always need a backup on your sump pump. There are some options to safely knowing your sump pump system is always working such as sensors that send updates to your phone, however having the peace of mind that a backup pump instills is nice. If you are going to theoretically rely on your backup someday when the main pump fails, be sure to buy a backup that pumps a reasonable amount of water. Someone more experienced will need to add information about the sock.
Back ups are always a good idea, but installing an alarm sensor is an easy 'catch all' solution. Backups can fail, but installing a sump pump alarm ensures that regardless of number of pumps, batteries etc. if something fails you'll know about it.
This is an example - http://www.absoluteautomation.com/sumpspot-wifi-sump-pump-alarm-for-texting/
One of the problems I have encountered with sumps in crawlspaces is that they tend to collect sediment much more than basement ones. Crawlspaces are much more difficult to work in than basements, so installation details such as fabric-wrapping and insuring there is clean #57 stone around the entire drain pipe tend to get neglected. The result is a lot of sediment build-up in the bottom of the pit itself. Then the pit dries out, sediment layer hardens, and process repeats itself during the next heavy rain.
Even if you have a stand-off at the bottom, it quickly fills up. This is where a much better sump pump itself comes into play, as it can handle powering through this sediment (within reason). The 1/6 HP pumps in particular are extremely susceptible to this type of failure, since they are too weak and their impellers are terrible. I have seen two of these pump installations fail in the past two years for just this reason. Both of them were Home Depot low-end models. If you feel this may be a problem in your installation, you may consider investing in a higher end model (Goulds, etc) capable of handling this type of abuse.