There are very few people (notably: meatheads) that are comfortable or happy to be performing the same exercise over and over and over. I can squat 4 days a week and be happy as a pig in poop about it but many athletes/clients would find that boring and tedious, which is perfectly fine. Not everyone has to like to train the same exact way that I do; what I eat won't make you shit.
This is where variations of a lift come into play: instead of
This is one of the easiest ways to alter a clients training volume/intensity without having to actually deload them. They still get a high training effect while working hard. But when does it just become too much?
As always, the answer lies somewhere in the middle. You have to have enough variations of an exercise that you can cycle through them and prevent the mental fatigue that a lot of people can get during long-term training. However, if you're cycling volume/intensity/variations appropriately then you really shouldn't run into this issue.
The problem that you can quickly run into, though, is trying to apply pretty ridiculous exercises to a client simply because you need a new variation. While I'm sure there's a solid benefit to doing band resisted pushups with your feet in a TRX and your hands on a stability ball and a weight vest while listening to Bring Sally Up and breathing exclusively through your left nostril, I don't think its such a significant benefit that you should take the time to coach it with your athletes. While it's imperative that you make your clients enjoy their workouts with you it is ultimately not your responsibility to keep them entertained with circus tricks instead of exercises. Get them an appropriate training effect and keep it moving.
I personally believe that each of your main movement patterns should have 5 or 6 barbell variations that you can rotate around. For instance, in no particular order - Hinge: deadlift, RDL, snatch grip RDL, glute bridge, TBDL, rack pull. Beyond that you can move to your assistance work where you can include a few more variations...even though those first variations are still legitimate. It's not a problem to deadlift and then do rack pulls as an accessory exercise! But you can also include things like KB swings, pull-throughs, GHRs and back extensions. That's a total of 10 posterior chain exercises to cycle through, and that's not even including things like manipulating volume, intensity and tempo of the exercises.
Admittedly, it can be fun to show off the ridiculously complex and difficult exercises that you learned in an obscure Russian manual written in 1943 and translated for the first time in 2011, but that doesn't mean it's necessary or worthwhile. Stick to the basics and learn to appreciate the process.
Have a great day and go lift some heavy shit!