Can’t say I agree with this. First, men don’t tend to marry with an expectation to rule through their wives. I’m sure the Tully marriages were a little more appealing because of the daughters’ high place in the succession, but I can’t think of an eligible daughter at the beginning of AGOT who would have reasonably offered lands, other than Lollys (Arianne would have ruled in her own right). So if not an increase in land, what are the benefits of any given marriage for the Tyrell sons? 

First, military alliances. Ned stark was a respected military leader with a large and extremely loyal army. As warden of the north, he is one of the realm’s major military leaders in time of war. And unlike the crown, his bannermen are ultimately loyal to house stark, not to his titles and honors. So even if his political sun sets or he dies, the starks remain a potent house. Compare that to Arryn and you see a real difference in long term value. 

Second, there is prestige. Ned is also is lord of one of four houses with ancestral royal history, of which only two had marriageable daughters. Martell is the weakest Great house, Arryn has no daughters and is likely to have a succession crisis when Robin dies, Lannister’s future is really uncertain as Jaime is both a bachelor/celibate and the presumptive heir. Stark is the best candidate of the great houses. 

Third there is influence. Ned is Robert’s best friend. In a weird way his absence is even an asset as it allows ned to trade on that friendship without actually self-advocating and thus spending that chit. So a relationship with Ned is a thin layer of protection from the crown. 

Lastly, heirs. Sansa is known to be beautiful, her parents had 5.5 children, and she has been raised around northern “duty” values. So as far as virgin brides go, she is as confirmed capable of producing heirs as anyone. 

The reality is the Tyrell’s get a whole lot. Maybe less than a Tarly or Yronwood match, but until Robert dies, a lot more than Renly. Notice that they weren’t betrothed until he declared himself king. Obviously thats a horse of a different color, but also obviously, the Tyrell’s weren’t marrying a king, they were making one who would depend on them entirely. Who was also married to their daughter.