I understand completely.
First thing is to understand how the application in question works. It's quite simple in theory...
The application on your PC connects to a server provided by Mikogo and from there attendees can view what you are sharing.
There are a few scenarios you should consider...
1. A hacker on your network can see traffic inbound and outbound and could see that someone was using this software. They could then start to determine your network topology and discover the PC in question. From there, they can craft a simple exploit that could give the user access to your PC and your network. Once they have done this, (here comes the scary part) they can find out how the PC in question connects to the network and then engage in a practice known as privilege escalation a process where by the hacker starts of as a limited user and then through various means escalates the account to administrator.
2. A hacker on the attendees network can see that someone is having a meeting and that the traffic is going to a Mikogo server. They then have two options... The hacker could break in to the Mikogo server and follow the route back to your PC, craft an exploit and do the same as above. Or if the application gave your outbound IP then a hacker could try bypassing your buildings firewall and would be able to do anything on the network they liked.
3. Mikogo suffers an attack and details of users accounts were compromised. I don't think I need to explain what they could do then.
While I believe your IT Manager is correct in his assessment, I would point out that the same could be done with messenger applications such as MSN/Yahoo/Skype etc.
It's this idea of being able to remain attached, accessing the system without any trace that has got me particularly worried
It worries a lot of people and I'm sorry to say it's a very easy thing to do.
1. Find target
2. Scan target
3. Exploit target
4. Escalate permissions
5. Cover tracks
I won't go in to the details of the above, but there are free tools out there that can do this very easily.
If you have any questions on this let me know.