Back in the DOS (1985 to 1990) days, the office I worked in started with 286s, (about 10 of them) that used Tandy DOS, 3.0 or 3.1, I think.
After that, we got 386s using MS-DOS 3.3 and 4.0 I believe, Dell, and as I recall, they weren't great either. They all had the same set of software on them, and depending on how many people used them, and how much they used them, they would start to lock up within about a week, if not rebooted.
We decided the best thing to do, was to Turn them on, first thing in the morning, and turn them off at COB, each day, which cured that problem.
Retired now, we have a pair of Intel DP67BGs, with Core i7 2600K processors, 8 GM RAM, WD 1 TB HDs, Nvidia GTX-240 and GTX 560 ti video cards, running Windows 7 (64 Bit) Home Premium, behind a D-Link EBR 2310 Router. But, we still live in the boondocks, with unreliable power, and the very good chance of some idiot skidding on the winter ice, or taking a corner too fast, that will wipe out a power pole or transformer, so we still turn them on when needed, and off again each time we are finished, to avoid damage when the power surges or blinks off and them on again.
My wife's sister has an older HP, directly connected to a hard wired internet connection, which she never turns off. However, she does suffer from odd lockups and data disappearing at times.
So, even with the modern Windows and hard ware, I still believe that there are sometimes things that pile up in memory (Stacks and other storage spaces), that eventually catch up with you, therefore I think that you should reboot any system, at least weekly, just as PM and for general peace of mind.