It depends heavily on your Gamemaster and the run. Some can be done without being present at all, while others the GM might be fine with the hacker not be present, and still others the hacker may as well sleep through the run for all the use they will be... and still other Gamemasters, like the ones that rotate through the game I play in, prefer to have the hacker at least near by on the run, and sometimes force us to go along.
Examples I often run into:
Signal Blocking wallpaper buildings. Our GMs love these tings. Mesh built into the wall paper and windows is a common one for any high scale establishment (to prevent AR spam from ruining the enviroment inside), not just high security ones.The diffrence here is that if you can get a singal inside, it doesn't tend to freak anyone out, it just requires some work to get through to the inner area. It's easy enough to get around by finding a weak point and sticking in some directional antennas, drilling a hole in through the wall/paper, sending in tapper drones, or clipping into a cable on the roof/sublevel/etc... but that's a lot of work (and sometimes risk) just to let the hacker sit at home, and the rest of the party won't often go ahead with such a plan unless they think it's safer to leave the often VR immersed hacker at home (or in the car) instead dragging him along on the run.
High Security buildings. Sometimes we get sent into buildings with real farday cages built into the walls, or underground complexes, etc. These require more work to get a singal inside, and even then having a strong signal bouncing through the building might get us detected because it's often looked for. If, once inside, the sysem that really needs to get hacked isn't even on Wifi... well now you got your adepts and street samis trying to uncoil cable, align lasers, and protect repeater drones through high security building in order to plug something into a terminal so I can hack it from home. Again, the rest of the group would rather drag me through the building then try and arrange for all of the above.
Off-grid zones. I've been sent out into the barrens, out into the woods, through a sewer system, up a mountain, under the sea, and to a half dozen other places that don't have a good signal. My hacker has a satelite uplink, but our GMs enforce the half-response lag time penalty for using the small portable kinds and the result is I'm better off going along. (I still get good use out of the Sat-comm for browse checks and the like when I'm out in these places, or for sending commands to distant drones/systems where the slight lag isn't an issue.) Sometimes we end up on runs where there isn't much to hack (like a run into Bug City) and on those the best I can do is be present and provide an extra gun, control some drones, and keep the the tacnet bonuses flowing.
Jamming. Sometimes we end up setting off an alarm of some kind, and the next thing you know we're being jammed. At those times I kind of need to be present to do much of anything.
Of course if the group has no hacker at all, the GM can gloss over such details most of the time. Similarly if the GM doesn't mind the hacker being remote, they don't have to make things so difficult. I have to say that the GMs I play with like to give me something worth hacking on runs, but they have plenty of ways of making systems secure enough I can't get away with too much unless I'm present or I convince the rest of the party to do some B&E to plant repeaters and links.
By the way, running an AI character means your hacker can be present (on another player's computer or in a drone) without having to be a VR focused non-corpse to be dragged around by the rest of the group. It's a similar trick to just having the Hacker ride along in drone wheelchair so they can be there without being immoblized. I spent most of one run on a Cyclopse Monocycle for example.
EDIT: QUOTE (The Jake @ Jun 22 2009, 06:21 PM)

I don't dispute you often need someone on the inside. My question is why must it be a 'hacker' as opposed to a 'street samurai with MBW and a Hardware skillsoft that knows how to string together some cable and hook up a relay'?
Running a relay isn't always the best or easiest option. Many guards will notice an optical cable being run through their building. Some Spiders will notice strange encrypted comm channels, and hiding laser links through high security doorways is often hard. It can be done, and I've heard some hackers prefer to do it themselves via a drone that travels with the group (rather then a street sami doing it for you) but sometimes it goes wrong and then your left with no way to make the hack as you're miles a way while your team is on their own.
As for a well equipped Sami with skillwire... well just give him a good commlink, load it with programs, and have him run hacking skillsofts and he might be able to do the hack on his own. Hacking isn't linked to the Logic atribute in use, so if he is skill(wire)ed with good programs he might just double as the party hacker. He won't be as good as a dedicated one with all the crazy hacker headware (simboosters, nanites, encaphalon II, etc) but he is viable.