They're huge on the civil service GL slingshot Optional mentioned (which can be pulled off pretty frequently on anything short of deity if you've got the start that suits it). Even if you don't plan to mass them and go offensive that early wonder rush will leave you quite weak militarily and likely to get a DoW. But finish GL and make a quick buy and even one well-used pike will absolutely crush a warrior/archer rush. Even if you don't plan to to a GL rush that top part of the tree is great economically and pikes are the only thing in there, so you use what you have. The difference in swords and pikes is pretty negligible other than the upgrade paths.
Also, as was mentioned, if you've only got 2-4 iron (which seems very common to me) I think it's better in the long run to spend that on siege and use pikes. It'll be a slow crawl if you have to face longswords with your treb + pike army but if you've got 4 siege with logistics in place already when cannons come on line you'll make up for lost time quickly.
I end up using them quite a bit.