The answer is - as always - it depends on your goals & likely in this case, budget.
[ MY VIDEO RESPONSE: COMNIG SOON ]
Let‘s start with the fact that ANY Beachbody program can be modified however the hell you want. No one can stop you, especially since Joel will be on the other side of a screen, not in your living room.
However, if a trainer, or any trainer calls for a piece of equipment, there's probably a pretty good reason why.
For the sake of our discussion though, let's just look some pros and cons to help you better decide what's right for you. Note, this list isn't comprehensive but I hope it'll help you make a better decision or consider somethings you haven't yet.
• Range of Motion (ROM) . Chest press, skull crushers, pullovers and many more, if you're stuck using the floor only, you're missing out on a lot of range of motion. Generally speaking "full range of motion will result in better muscle balance, joint stability, proper activation the working muscles and over better movements quality" says Sam Simpson, CSCS, CPT, co-owner and vice president of B-Fit Training Studio in Miami.
• Isolation. Standing up while lifting often leads to unintentional body swing. Even the most advanced lifters have trouble eliminating this. Simply put, the less muscles that have the option to move, the more likely you'll be isolating the ones you want to work.
• Options. More equipment equals more variations. This is one reason we pay for gym memberships. Hitting muscle groups from different angles can help with overall development, different stimulus and it helps eliminate boredom.
• Focus. For LIIFT More the cast will all be using the bench so unless you want to invent your own variations mid-workout, stay focused on the weight you're trying to lift, not trying to become a trainer.
• Budget. Benches aren't always cheap. While not a huge investment for most it's still an investment.
• Space. If you're already tight on space, a bench doesn't help. Although my clients love to sit on mine while I'm yammering, not many bench fold up or store easy.
What About Using a Ball Instead?!?
While I applaud you trying to find some middle ground here, this still isn't really an option for this program. The stability ball might work a little for flat bench exercises but for incline exercise or even step ups, you're left still missing a lot.
If you’re okay trading off ROM and some brain power making your own exercise selections then no, you don’t need a bench and you’ll still get a great workout in no doubt. I think for most, it might seem like a good idea but just a few workouts in, you'll be regretting it.
My personal opinion - buy a bench. After dumbbells a bench is a staple of any home gym. It’ll last you a long time and will likely lead to quicker results.