#2
You can certainly do it.  
I don't, for two reasons. One, it requires more hands than I usually have available and a lot of squinting at a screen and no real way to change something quickly if I want to. Two, it requires me to put at risk an expensive laptop. 

If you want to save money, buy a used Pod XT (I've seen them for under $80) and maybe an FBV Expression pedal (used about $50). The software may be cheap/free but busting your laptop at a gig is NOT going to be cheap. Besides, you still have interfaces to worry about, outputs to deal with. I'm not sure there's really an upside to doing it this way. 
#4
i've been using amplitube live for about 5 years now, as my exclusive rig. i play live 1-2 times a week, requiring a variety of sounds.

macbook, interface, fcb-1010, and a tuner pedal works well for me.
#5
DJ acquaintance has gone through four laptops in the last 18 months, trashed for one reason or another.  I think he's looking at something "hardened" for the military next. I decided I don't want to be at the events he hosts <G>.  He needs to use a laptop because everything he does revolves around it, and because it's a lightweight system compared to the old setups.   

I've also got another acquaintance who does shows in which he provides most of the music live (keyboards) and manages other stage effects on a laptop he's got on a stand next to his keys. He keeps three laptops, maintaining any changes on all three, in case one goes down. 

Here's another consideration: I take backups with me on paying gigs (who cares about open mike nights?), and a backup in this case would entail everything from guitar to the point of hooking into the mixer (at that point it's up to them).  For me that means identical pieces (I don't want to be put off my feed trying to remember what I can't do because I've got something underpowered as a backup), and for a laptop setup that means all those bits and pieces as well as a backup laptop with everything installed and programmed. Otherwise, it's, "sorry, ma'am, I'm not going to be able to play your wedding song because my laptop lunched." 

If you're one of those folks whose backup for his Axe-FX II XT SR GT is a Spider II, fine.