The short answer is YES. Followed by a large “But.” Your legs will get stronger; your horse may just become more dull. Lots of times strength isn’t the issue because as has been said many times, it’s not the volume of pressure on the horse’s sides (witness their non-reaction to the girth) that matters. It’s the changes in the pressure which convey meaning.
I am certainly in favor of a rider who has the capacity to be strong. Riding without stirrups can improve this too, but be aware that some horses at some levels aren’t appropriate to be ridden without them. A horse that needs to go forward and find his hindquarters doesn’t need you impeding it with sitting trot. And as for posting without stirrups, that’s a H/J thing that’s great for grip but not so useful for aspiring dressage riders. Just forget it!
I mentioned “the capacity” to be strong. That’s to remind you that applying strength all the time is counterproductive. What you get is a codependency, and who needs that in a relationship?
Steffen Peters reminded us at a Trainers’ Conference that the spur should never be used to sustain a movement. It can sharpen the horse’s attention and responsiveness, but then it must cease and allow the horse to carry on, even for just a few steps. Whether you use spurs or not, the idea is to deliver a meaningful message with the natural aids, reinforce it as necessary with spurs and/or whip, and then give the horse a moment to translate your wishes and to carry on without being nagged.
If you are used to wearing spurs, it’s a good idea to take them off periodically in order to see “how much horse” you’ve got under you. On the other hand, don’t get sucked into being such a purist that you think it’s better to rely only on natural aids even if what you’re creating is a horse who tends to smirk when he feels your demands.