You can if you want. I've had mine still in place but split up with an isolator for about 3 years out of the 4 since we bought it brand new. Never had any problems with the set up, except for issues with the isolator not charging the second battery....
I've never had any starting issues with just the single one, recent!y replaced it with a century ns70, more as preventative maintenance. Did a load test on the starter and it was still OK, but as I drive a truck away for work, sometimes it can sit for a week or so at a time. Last time I got back it was fairly late at night and didn't sound real convincing, so I replaced it so I'm not stuck late at night waiting for a jump start....
In saying that, 4 years out of only one of the standard batteries starting the engine was a pretty decent effort I reckon. Now with the new one though it kicks over like anything... Just gotta replace the second one now, looking into a dc-dc charger to replace the standard isolator and probably an agm, not sure yet whether optima or century or another brand as I haven't looked too closely at it, still saving for a canopy before I put a new second battery in.....