The grout between the vertical tiles in my shower has cracked and opened.

I have repaired it a couple of times already, 10 to 16 months in between.

Is it ok to use silicone instead of grout to seal the joints?

There is silicone already at the base, along the joint with the ceramic shower tray, and at the vertical corners of the shower cubicle. This silicone is almost intact, sealing perfectly and with only a few mold spots after more than three years.

It may not look "conventional" but it may produce a better seal. I don't care much about the look.

I need a good seal because there are water leaks through the cracked grout.