It could! Some of us used Texas TI-51. So there, HP-freak :-) ...

Even if you allocate all memory in the initialization phase and even if you do not use any dynamic tricks - such as recursion, dynamic function calls or the like - there could still be a problem. If the programmer is a bit sloppy s/he could forget to initialize a variable (for example a pointer variable). If that variable is used only in certain situations that could cause a problem then. Another common situation is not to handle all exceptions. An given exception may be very rare and thus forgotten about, until that day of doom when a situation occurs that throws the exception. If that one is not handled or handled in the wrong way we also have a problem that will cause the state of the modem into unknown land. Suddenly a variable may have a new value that stopped parsing or whatever. Communication devices must be designed to deal with any kinds of interrupts at all time. It is complicated to test if the software is stable under all circumstances and the cheaper the device is, the lesser time to test is give the developer. So, lack of testing could also be a problem. It all leads up to the situation:

1 In the beginning - all is fine
2 In due cause something happens
3 That something changes the state of the modem into uncertainty
4 Now the modem is behaving "strange"
5 So: Reboot it and go back to 1.

That is my take on why you should reboot your hardware from time to time...