I used to think so, but I've been playing sans-drummer in a trio (Pedal Steel, Acoustic/Vox, Bass) for about 9 months and I am loving it. At this point I'll take a Pedal Steel player over a drummer almost any day.
My experience is that when you add a drummer to the mix, alot of stuff is added.
-There's the difficulty of finding a good, reliable one.
-You typically have to rehearse because sticking to parts and nailing breaks becomes much more important when not all the members have direct eyeline to the leader.
-You have to split the $ with another member.
-Unless you have a drummer willing to play extremely quietly (hard to find) you're generally limited to clubs that have a PA and the acoustics that will handle louder volume.
Without a drummer we are a bit more at the mercy of the leader as far as tempo, but I've gotten pretty good at steering the ship and as far as adding verses, extending songs, mixing it up to follow the moment. etc, we can almost turn on a dime. With just 3 of us, we can tote a mini-PA to play lots of small gigs in places that have no PA, there's no need for rehearsal and with only 2 guys along, the bandleader is able to hire me far more often.