What are the most commonly-held views on flying more than one make and
model of aircraft?

Take the example of a 172 and a Warrior. Clearly the aircraft are
different and the best solution would be to fly only one make and
model, but the reality of plane rental is that if you are checked out
and current in more than one aircraft, you have more flexibility in
terms of when you can go flying, which means more fun and more overall
currency.

In my vast career of 63 hours total time, I've flown several 172s and
Diamond DA20s, a 150, and a Beech Sundowner. Flexibility is fine and all,
but for me it's just a lot more fun knowing how to fly more than one kind of
plane. Also, it's gotten me a lot closer, faster, to knowing what kind of
plane I want to fly long-term. As far as transitioning goes, I found that
the differences in radio and panel layout and procedures were more
noticeable than differences in flight characteristics. The instructor who
checked me out in the DA20 kept saying, "Don't worry, it's just another
airplane," and he was kinda right.

At what experience level is it generally seen as sensible to regularly
fly two different makes and models of plane? Is this a big deal or not?

I don't expect there's a right answer for everyone, but I didn't want to try
something else until I felt comfortable enough in the 172 I trained in. The
reason for wanting to reach a certain comfort level before moving on was to
have something reassuring to come back to -- nothing more sophisticated than
that.

-Tom Young