I've been a bit around the block and here is my story.

I used to produce a lot of music in my home studio, so I wanted to bring those exact sounds with me live. For quite a long time my setup consisted of either Logic Pro, some softsynths and a midicontroller. Later Logic Pro was replaced with Mainstage. The possibilities where endless. So was the risk of errors.

I found myself spending more time tracking midi cables and setting up controllers that had somehow been mapped wrong than actually playing music. My band was always making fun of me because I could never get the right sounds on time and generally while the setup sounded better than most digital hardware synths of the time, it was too much of a pain to maintain and prepare.

I felt like I used huge amounts of time on this, but being honest with myself I've never been the kind of person who prepares a lot in advance and most of the time was probably spent very inefficiently. I was often trying to set up stuff while the rest of the band was waiting and often making poor decisions due to the stressful nature of the situation. If I had spent some of that time setting things up in advance instead, I would probably have had better results.

Then at one point I bought a Nord Electro 2 and quickly fell in love with how easy it is to program on the fly. I hooked it up with an extra keyboard to be able to play with two manuals and relied fully on it for my base sounds ...well, until I needed a synth patch. Suddenly I had to wire Mainstage back into the mix and issues immediately started cropping up again.

So I sold my Nord Electro and went out and got a Nord Stage instead. That was the best purchase I have ever made! It is a 1st gen so it is getting pretty old, but the fact that a new Stage costs so much and this one sounds so good still means that I cannot justify buying a new one yet. I still play the majority of gigs exclusively on this board. The philosophy is the same as the Electro. It is quick and easy to program on the fly. Obviously I have presets for most songs, but if we are rehearsing a new song and I realise that it would be nice with some organ over the chorus, it's just a matter of pressing two buttons. Something I never was able to achieve with a computer based setup.

Another thing is the ability of a sound to cut through a mix. In a live situation there is usually not a lot of time to do anything else than levels mix-wise. Whenever I try software piano libraries, they sound great on studio monitors and headphones, but once they are playing through a PA they sound like shit and really can't cut through the mix. The sounds on my Nord was designed for this and they almost always work on any PA without much EQ'ing.

If you prepare really well, I'm sure you could run everything off a laptop with good results. But there is a lot you need to think about. First, you need a good audio interface. Running a 10-15m minijack out of your computer wont cut it. Then you really need to work with setting up a good midicontroller to your preferences. You probably want to map the controls so you always have fx. filter cutoff on the same place. Besides having "full patches" with all the sounds you want for a certain track, you probably want to do some template patches with some of the things you commonly use (i.e. a piano/synth split, maybe piano and strings layered, maybe a bass synth and an arpeggiated synth) so you can have some degree of instant gratification when playing rehearsing. You have to think about "mixing" your live mix on a PA, figuring out where the problematic frequencies might be and EQ them out and you probably have to apply some compression as well to make everything sound even. Also, it is extremely tempting to keep adding extra layers and synths "because you can", when you have a setup like that. Don't do that. Everything you add to the setup is a potential source of errors and as you mentioned yourself - it's not much fun debugging a computer on stage.

In terms of having a powerful computer, that certainly helps, but I started doing this 10 years ago and I was ok. Computers have obviously evolved a lot since. Avoid the crazy fancy new top-of-the-line softsynths that are usually CPU hogs - and go with something leaner. I used to mostly use the built-in plugins in Mainstage and maybe a few specialised plugins.