Well, turntable inputs are designed for low voltage signals, much lower than "line level" inputs like cassette, CD, DVD, etc ...
So first, the TT input levels would be overdriven by other devices.
Second, TT inputs have an RIAA curve to compensate for the curve used when recording the vinyl. Think of it sort of like Dolby is for tape. Even if you pad down your auxiliary signal to run it into the TT input, the signal will get distorted from the RIAA curve.
Best to get an external switcher if you need more inputs.