lil brown bat

I guess I'd say that crosstraining can help, but only if you sit down and define what you mean by "meaningful gains". Muscle strength and endurance potential as measured by what? For what purpose? Do you want to be able to lift some arbitrary amount of weight, or do you just want to generically be "stronger" and have better "endurance", or is there some specific thing that you want to do (climb some mountain, go on some trip, take up some new sport) that will require greater muscle strength and endurance potential?

I'm a believer in functional fitness. My motivation for getting fit is to be better able to do things I like, such as skiing, aikido, tennis, whitewater kayaking, hiking, etc. As such, those aren't "crosstraining" for me, they're the goal. So, my "crosstraining" is quite a bit different from someone who starts out with activity A but then "crosstrains" in activity B for some ultimate goal that has nothing per se to do with either of those activities. In a nutshell, if your goal is "fitness" for its own sake and as defined by some arbitrary criteria, you're looking for one type of workout; if your goal is performance, you're looking for a different (functional) workout.