i have always tried to try as many operating systems as possible. My limitations are hardware really. at the moment, i've got slack and win2000 on one machine and xandros and win98 on the other. even though i don't actually use those windows systems, it was a conscious decision by me to make them different ones, so if i ever do, i'll get variety. I have had all sorts of linuces on these machines, SUSE, Mandrake, Knoppix, RH/Fedora, Turbolinux, Lycoris, BasicLinux (which i really liked actually), Peanut Linux (which i also really liked) and so on. I also had windows 3.11 for a good while. i was trying to get it to *work* as one objective (i failed, to get it to work with the sound card, the cdrom, the printer et cetera) but i also had a fun time using DR-DOS instead of MS-DOS (my advice, install win3.11 over MS-DOS first, then install DR-DOS over ms-dos afterwards, DR-DOS installs extra stuff, if it detects windows!) and replacing everything with FreeDOS, GNU tools or other Open Source win3.11 stuff. great fun.
I'd dearly love to get another machine and dual boot it between BeOS and Solaris too. I have never tried either and would love to try both or either. I just haven't got the disk space at the moment on either of the existing machines. Dual booting's fun too, i have no idea what the multibooting facilities are like in Solaris or BeOS so maybe i would have to resort to making a small linux system go on there too, just to use GrUB. I'd like to try some BSDs too, i have the install CDs for FreeBSD 4.5 and 4.7 but haven't got to play with them because of no disk space, i can't make them function to do all the things the other users in the house require and so i always have to install a linux over the top of them after about a day
so in short, yes, use as many as you can. Even installing slackware as well as red hat will make you think a lot more, what with the BSD init, and all the other different defaults (if installing slackware, go and edit /etc/profile as soon as possible by the way, since by default it puts . in your $PATH, if you'd believe that!). I still haven't got the USB printer to work under slackware (and am going to buy a parallel cable (which luckily the printer has a port for) to use instead as a result) but it's all part of the fun...