How About Hearts?

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Q 10 9 5 4 3 2
A 10 5
A Q 6
5 4 J 8 7 6 2
A J 7 6
J 8 7 2 Q 6 3
10 9 8 4 2 J 5 3
A K Q 10 9 3
K 8
K 9 6
K 9 4

NorthEastSouthWest
1 Pass 1 Pass
2 Pass 4 NT Pass
5 Pass 6 All pass

NorthEastSouthWest
1 Pass 1 Pass
2 Pass 3 Pass
3 Pass 4 NT Pass
5 Pass 6 NT All pass

This hand would be at home under No Trump Slams,, and indeed, I decided to put it there also. So a discussion of why the no trump contract should have been found is given there. Here I am only interested in the first bidding sequence, and the totally inexcusable insistence on a spade contract on a somewhat less than self-sufficient suit in the face of a heart fit! This not only gives you a bad score here, but I've gotta suspect endangers the partnership relations. The partner tolerant of goofs and misapprehensions here and there may not be so tolerant of a partner who tends to hog the bidding and insist on his suit rather than show support for his partner's.
The two heart rebid should show (at least) a six-card suit. The reason it should show six cards is that if you have only five hearts, cannot support your partner's spades and don't have a four-card minor, then you must have no trump distribution, and a rebid of one no is indicated. Of course, the calculus is thrown off if the bidding goes one heart, pass, one spade, two diamonds! Declarer may have no convenient bid other than 2 hearts even with only 5 cards in the suit, so when the bidding gets crowded, partners have to remember that partner may not have had the most convenient rebid open to him. Of course, one can pass two diamonds with a minimum, but with a couple of points above a minimum, one wouldn't want to pass.
In any event, the doubleton K 8 of hearts is fine support opposite the promise of a 6-card (or better) suit. You don't need to name the suit with high cards trump. Those high cards will (almost surely) win tricks in a heart contract. (Actually, only one top spade is cashed, sluffing a diamond, but then you don't need any more.) What you're looking for is a fit, well a major suit fit, of at least 8 cards. Not finding that, there are, well, three obvious choices you might resort to: a good minor-suit fit, a 7-card major, and of course no trump. But finding that desideratum, you'd do well to name that suit trump -- unless, and particularly in matchpoints, you feel secure enough in the other suits that you can bring home just as many tricks in no trump.
What if North has only six hearts and three spades? Then after the delayed support of hearts, he can show a delayed support of spades, so that you know he doesn't have four (or shouldn't have). And if he doesn't show that support on 3 pieces and you're in a mediocre 8-card heart fit instead of a powerful 9-card spade fit, then of course, it's the South player here who would feel aggrieved. And so it goes.
Still, I would strongly recommend getting your bidding right and accepting the grievances that allow you to look elsewhere for a partner or stick with this one as you choose rather than inducing a grievance in your partner who decides to drop you without appeal. Please bear in mind that this is not to suggest that one partner or the other must suffer a grievance. On the contrary, if you get your bidding right and partner gets his right, then there is no need whatever for either to feel aggrieved. But you can't both get your bidding right unless you get yours right. That's all I'm saying.
In any event, there are few things that pave the way for harmony between partners better than an immediate announcement of support for partner's suit when you've got it. Don't corrupt anything so as to show that support. If it's not there, it's not there. But an 8-card fit is traditionally the level where you can control the hand, get out the trump and cash your side-suit goodies, and South should certainly not have suppressed knowledge of that fit (which actually turns out to be a 9-card fit, of course) for a six-card suit without any indication of a fit from partner.
In my lexicon, an eight-card fit is a "modest" fit that cannot compensate a whole lot for a shortfall in hcp's, but still a fit usually strong enough when you have a goodly supply of hcp's. A nine-card fit is a "good" fit, obviously better than an eight-carder, but still not an unlimited supply as some declarers seem to treat the 9-carders. It isn't until you get up to 10- and 11-card fits that I speak of superfits where you want to give serious thought to naming that suit trump so as to exploit that fit.
The moral here is: tell your partner of support that is consistent with finding 8-card or better fits!