The more "load" you put on the Server, the slower it will run for each. You may not be able to predict how much these other apps plus FileMaker will slow down response times until you you use it, however.
The risk with any multi-application server--whether you use FileMaker or not--is that a bug with anyone one application will bring down--possibly crash--all the applications currently executing on the server.
Sorry but FMS11 is not extremely stable. I have numerous deployments on both macos and windows and in both kind of environments the fmsadmin-console either stops working or start to react extremely slow.
There is no difference between the deployments where the server is also serving other things like files, sql-servers, webserver etc. and dedicated fmservers. MsExchange is an exception here ... fmserver and exchange do not mix well on one server.
The trouble with the console set aside (the server itself, including running backups, runs just fine), you need to consider teh capacity of the server fms is running on. In a lot of (Mac-)environments it seems customary to use non-specialized hardware as server-hardeware. In such cases it is wise to dedicate your server to running FMS.
If your server has an i7 processor and plenty of memory it would be like throwing away money not to deploy other services n it. There's no need to immediately restart your server if a little thing goes wrong nor will your server hang if one of the apps crashes ..... However all servers have a GUI and most of the time the problems that occur find their cause in the GUI. Everyone remembers using an fm-client where a mousedown within filemaker would force the CPU to go up to 100%.
In short: You cannot say that running fmserver on a server that runs other deamons/services as well is a bad idea, but you also cannot say that it is a good idea ;-) It depends on hardware (CPU, memory, disks etc), choice of OS (server/non-server), need of capacity (are you alone or with 100 users) and the knowledge of the person maintaining it.