Why progamers siege/unsiege all tanks at once?

It seems to me that it's a common practice both in sc1 and sc2 to unsiege all tanks simultaneously, relocate them, and siege again. I wonder why it's so frequently done instead of splitting tanks into two groups, with one group always covering another?

I see two main disadvantages of this approach:

• it can slow down terran advance (but it all depends on relocation distance; sometimes all tanks are usieged and moved to a short distance only to siege again; with the same average speed you can move half of the tanks twice as far)
• it take more apm

On the other hand, main advantage is that you are unlikely to be caught unprepaired, and even in such situation, it would be hard for your opponent to determine that tanks in behind are actually unsieged, so chances are that he decide to fall back, taking casualties under your front tanks fire.

So I'm really surprised to find this tactics so rarely used. Am I missing something?