When I tried one out it colored my sound when it was disengaged, a deal-breaker for me - if I turn something off, I want it completely off. I would have had to use a looper to route around it, like with some old pedals.
I can think of no good reason for this.
But the SCR is a relatively older pedal, I would think this would be rectified with a newer one like the SGT.
The SCR-DI isn't a straight in/out pedal and it makes sense because you have an aux in, input, thru, line out, headphone out, and di out. The unit was designed to drive an amp, feed a signal to a mixing board, or be a portal unit for pratice. These type of full featured DI's (Ampeg or otherwise) aren't pedal board mainstays if you want an unaffected tone. However, the stand alone Ampeg classic pre and scramblers are true bypass because they're not taking or feeding more than one input/output. The stand alone until would be your best option if you need the pre or scrambler but don't need the other options. I wouldn't hold my breath for such an option for the SGT-DI since it's relatively expensive and feature rich in the pre section alone.