Source: Hand Evaluation – the power of ten By Andrew Robson
When Mr. Milton Work invented the point-count method, ace = 4, king = 3, queen = 2 and jack = 1, he did the ten no favours. Contrast these two hands:
Both contain the same shape and the same jacks, queens kings and aces to make up 12 points. But what a difference!
You would open 1NT with the first, but with huge trepidation. If partner raises to 2NT, a response that invites you to bid again – to go on to 3NT with an upper range hand and pass with a lower range hand – you couldn’t pass quick enough. You’ll probably not make 2NT, let alone 3NT.
The second is a completely different kettle of fish. Your hand is replete with tens – those minor-suit nines are pretty good too. You’d raise 2NT to 3NT quick as a flash, as your hand is worth (at least) 14 points. Examine the diamond suit and give partner, say 
J5. Q642 facing J5 will probably generate no tricks at all; Q1093 facing J5 will generate two tricks: for sure. What a difference!
Milton really should have given the ten half a point (or perhaps 0.4) and the nine about a quarter (or perhaps 0.2), but that’d be way too complicated. However don’t forget how useful these intermediate cards can be.
Source: The Power of Tens by Brian Gunnell
Put yourself in the South chair. Your partner opens a 15-17 1NT and you have to decide whether to invite game, or whether to bid 3NT all by yourself.
The usual benchmark for going to game unaided is “a good nine or better”. One thing that makes South’s nine-count not so good is the square shape (3-3-3-4), you can almost subtract a HCP for that alone. Does this make South’s hand a “bad” nine? No, not at all, look at that awesome array of intermediate cards. All those lovely tens may not count for anything on the 4-3-2-1 HCP scale but they certainly count for something in the play of the hand, especially in notrump contracts. So, South disregards the dismal distribution and, rejoicing in those splendid tens, goes directly to 3NT.
Declarer has a minimum 1NT opening (and would have declined a game invitation) but, even so, 3NT is a fine contract. The black tens protect declarer against the enemy suits and the red tens help declarer to build tricks in his own suits. It will be ten tricks on most lines of play, but check out what happens if all of South’s tens are swapped with East’s lowest spot card in each suit. Now it’s hard to see how declarer can scrape up more than seven tricks. Here’s to those tens, the most underrated cards in the deck.