I'm a big advocate of lifting HEAVY to develop a strength base and also correct postural issues in beginners. By heavy I mean the 1-3 rep, close to your 1 rep max. But the key here is the focus is on form, not the weight itself. Most average people can't perform a full squat or deadlift due to weak posterior chain, tightness in the hips and other widespread problems. Going through the process of developing these lifts, and doing the necessary stretching and supporting exercises to perfect form does fantastic things for the body. Beyond even boxing skill, these things make you a healthier person. They're essential movement patterns. Once form is mastered and the person can lift a respectable amount, I do think the diminishing returns set in very quickly. But enough can't be said about building up a foundation of strength in athletes. It sets them up to be injured less and actually cures a lot of technical issues just by fixing the way the body moves.

The most common misconception here is that lifting heavy makes you big and slow. Neither of those are true, for the same reason adding long distance running to your regimen won't take away your explosiveness. You just have to be aware of your diet, and understand that it won't be the only thing you're doing. You might feel slower while you're lifting just because it's taking energy that you weren't spending before away from your boxing, but once you're done you'll be just as fast, stronger and likely more mobile.

Now if you're really against lifting heavy, you can get the mobility benefits from just training those movements without weight or with light weight. As long as the focus is on perfect form and making the body move the way it's meant to through it's full range of motion you'll get a lot out of it.