If I play, I sub someone in.
It's really good training and the macros that I've developed as a result of it help my core team too.
I'm a prot pally, 3 elemental shamans and 1 resto shaman. No matter what I bring in I've gotta be quick on re-writing a few macros. Something I learned was that I could keep a spare set of macros around for everyone I ever PuG with, but that takes up a hell of a lot of room. So better I just add /target whoever into the macro for my offline member.
Yeah yeah I know, this wasn't a question about macros and stuff, but here it comes anyway. Here's my team:

Pomme (tank)
Sangarde (Alpha female DPS)
Snew (DPS)
Tweet (DPS)
Shingo (healer)

I don't know if your server is like mines, but even though we're not a PvP server 99% of the server is DPS specced folks. So chances are I'm ditching one of my DPS to bring someone along. That means either Snew or Tweet get replaced.
I needed a way to not only heal the person if they're a meleer, but also if they're ranged and can't just mooch chain heals off of my tank. So it's like this:

Sangarde has a HEAL SNEW macro:
/target snew
/cast [modifier:alt] chain heal; lesser healing wave

In fact Tweet and Shingo and Snew have it too. Shingo I just add a /castrandom and riptide in because he's resto spec. If I hold down alt while I hit it, all of the shamans chain heal that target. If I don't hold down alt and hit it, all of the shamans lesser healing wave that person.
So let's say I invite Doomchickenstein and Xkcd along because they have leet boomkin and DK DPS. I'm gonna kick Snew and Tweet. Instead of moving macros around, or making special ones, I'll just add into the old heal snew macro one more line.
/target xkcd
/target snew
/cast [modifier:alt] chain heal; lesser healing wave

Now since Snew's not online and I'm grouping with xkcd, I'm just gonna hit that macro and it'll heal xkcd. Later when he says "g2g I hear my mom coming upstairs kkbyelol" I just boot up snew and without having to swap any macros, she's back in the healing party. The computer won't target xkcd because he's now off in bed pretending to be asleep. Just like it wouldn't target snew while I booted her. So same thing for the tweet macro, I add in one line on all the shamans for Doomchickenstein which takes less than 30 seconds total.
/target tweet
/target doomchickenstein
/castrandom [modifier:alt] chain heal; lesser healing wave, riptide

okay this time I showed you the resto version in case anyone wanted to ask. If tweet's offline it'll heal whoever I put in there. If she's online (and they're not) it'll heal her and not them. Hold down alt, and it'll chain heal them (for whatever reason; usually because I like making green layzor beams)

Back to the subject, like I said: It's great training to work with different types of people. Rogues, whirlwindy warriors, DKs who death grip stuff, druids who hurricane as soon as you pull - you can't know how to deal with everything that goes wrong until you make a few mistakes! So don't be afraid to, it's worth it. They're usually greatful to have someone to run heroics on their new alt with anyway.

-Mooni