Stage Weapons Should Not Be Used In Sparring.

Their construction and materials are generally less damage tolerant than their "battle ready" equivalents. This runs the risk of several bad things:

• The stage sword breaking, possibly harming you or your sparring partner.
• Lack some safety features (rolled tips, increased flexibility, etc.)

Additionally, they lack veracity. They are too light, have an odd center of balance, and only look the part. Don't use these for sparring!

The reason steel feders are so widely used in HEMA is due to them being able to be used safely in partner drills, especially at higher intensity .

Stage combat swords miss the rolled tip and bending in the thrust, instead many of them have rather pointy tips. This makes them really unsafe for partner drills and there have been a number of accidents where people ended up with stage combat swords penetrating their body in very unpleasent ways.

To be totally frank there have been instances of even feders stabbing through hands with open palm gloves, but considering their widespread use and the oftentimes higher level of force when used, those instances are much much rarer.

Also stage combat swords are balanced for stage combat and not for fencing and while some of them have decent balance and handling qualities, many of them don't. Add to this that most people like to use the same sword for solo drilling that they use for partner drills or don't want to buy a second sword for the same purpose / weapon system, it becomes clear why feder use is so widespread.

tl;dr As long as you're only doing solo drill its okay to use a stage combat sword if you like it's handling and you have the money to spare for the luxury of having two swords filling a similar niche.