Xbox Live Silver account can't play multiplayer?

I've been thinking about making a secondary LIVE account, but just leaving it Silver. I've always heard that Silver accounts can't play online, but I swear I've seen Silver accounts playing online. 
 
Does anyone have any experience with Silver accounts? Do you just get denied access to the Xbox LIVE component of a game?

Perhaps they just used a trial (like the 48-hour cards you find in certain games) or perhaps it was a special weekend to promote a game/ online gameplay :P 
The accounts will still be and look silver in both of these cases, (edit:) I THINK

You're not able to play games online unless you have a 48 hour pass or something like that. The game will tell you that you need an xbox live-enabled profile to access that.

Ahhh. Maybe they were using one of those. God that's annoying.

I'm curious, is the second account also for yourself? For what reason would anyone want to do that? Are you living a double life, man?!

" I'm curious, is the second account also for yourself? For what reason would anyone want to do that? Are you living a double life, man?! "