Well, I'm not saying that a visual inspection will tell you with certainty whether there's a latent infection.
What I'm getting at is that if you've taken care of your tubing and have either washed it promptly and thoroughly after use, or have soaked it thoroughly in a good cleaner, it's probably fine to reuse. Making the (hopefully both reasonable and obvious) assumption that one is going to sanitize the tube again immediately before use, I think this is reasonable.
If there's a reason to think this isn't the case because of a scratch or some trub that's caked on and won't come off, then a surface sanitization before use isn't sufficient. It may not penetrate a scratch, or the trub may dissolve during racking and expose an unsanitized surface.
So if there's a reason to suspect that is the case, then replace the tube. If not, it's not much riskier than using fresh tube I wouldn't fault anyone for replacing tube more often than that, but I don't think it's necessary.