Why do the books say NOT to play 34suited? thx

That's a good question.

I always thought it was for the straight potential.

4-5 can make a straight with 3 cards on either end (A-2-3 / 6-7-8) whereas 3-4 can make the straight with 2 cards on the low end. You give up a card. Dunno the math behind that being worthwhile or not though.

Anyway, I'm sure it's not because 5 holds some awesome power that 4 doesn't...

I think the straight and flush potential of 34 suited is weakened by the fact that you will never make the nut flush and only sometimes (2/3 of the time) make a nut straight. 45 and above have one more straight combination that is a nut hand. Really doesn't seem like enough to play anyplace but late (probably as a blind steal attempt in a tight game) or for free / cheap from the blinds. Most hands you make (Flush and 1/3 of your straights) you can't push very hard and therefore you can't make much if you do hit.

If you hit your flush, bet hard and get pushed in you really aren't that happy. I think the straight hands you can make are better hidden and more likely to get paid.

I'm a novice but thats my 2 Cents