It's pretty easy to break a switch to get it to send you someone else's traffic if you try. Or even if you don't, honestly. The isolation provided by switches is much more about bandwidth use than it is, strictly speaking, about security. Simplest case: under normal conditions, a switch will blindly trust anything plugged in to tell it what it is; and if you tell it that you're another device, bingo, you'll get all that device's traffic until that device reasserts itself.
That said, you have to decide your own risk profile. If this is literally just an addendum to the Mac, and you're not really using it for anything else, telnetting into it is probably fine. If you're also using it for other stuff, or reusing passwords, you might want to be a bit more careful.
(edit: meant to say, I am agreeing to a great extent with
@AndiS here, although the idea of someone having a switch that accidentally has an SNMP implementation that actually
works made me snigger)