mIRC scripting is single threaded, which means everything is done in order. So if you had an identifier that requested information from the server and had to wait until it received it before continuing on in the script, that would cause everything else to freeze until it completed. If you know you can get a response almost instantly every time, then an identifier could work well. But with a server, it could take a few seconds or even a few minutes if you have bad lag. It could entirely lock up mIRC until you break out of the script if you get disconnected and don't get a reply at all. That is why an identifier wouldn't really work for that kind of situation.
You should be able to do whatever you need with RAW events. More detail on what you want to accomplish might make it easier for someone to give you more exact information on how to do it. However, what Loki showed is a good start for what you probably need. It also shows how you could trigger an alias directly from the RAW event so that it does something once you receive a reply.
You have a variety of ways to save data in variables. For example, you could /whois a nick and using RAW, you could set a variable that showed the last /whois's channel list. Then, you could use that variable whenever you want. Of course, that only gives the last call. You could manage it another way by using dynamic variables to track more than one /whois at a time, though that can get a little complicated when using multiple RAW numerics.
