I would like to begin by explaining that selecting the best travel credit card is different for each person depending on where you fly, how you fly, and what you want from the card. For example, do you tend to fly business vs. economy, do you travel to the Middle East, do you want to save up all year and take one big family vacation at Disney? There is a card that is right for everyone.
The most important thing to do is figure out the value per point; e.g., the range typically is 1.4 to 2.2 cents per point. However, the value of points is relative to each user and that's difficult to calculate. If a flight would cost you $350 but you use 25,000 points, this is 1.4 cents per point. I typically only use points for flights of over $500. The Chase Sapphire Reserve card in the U.S. tends to offer the highest value per point, but has a high annual fee.
A bank credit card, like Bank of America Cash Rewards offers 3% on gas, 2% groceries and 1% on all other purchases, but there is a cap of $2,500 spending in these categories each quarter. Earning cash back is the most flexible program, but some people like having to spend points on travel as it makes them travel.
Other people use points for rewards that are "priceless," such as invitations to special events (access to places or people others do not have).
Does it make sense to get even the best travel credit card if you only travel once or twice a year?
Yes, if it makes economic sense (how much do you spend on credit cards and will you earn enough points to book travel plus consider annual fees, and the amount you need to spend on the card to earn the bonuses offered).
Is $1 in cash back more valuable than $1 in a hotel's/airline's points/miles?
As answered above, the 2-3% cash back is more flexible and could be more of a value, but there are caps. For cash back, if I spend $100 on gas, I get $3 back. For a points reward card, if I spend $100, I may get 200 points (2 points per $1 spent). If each point is worth 1.5 cent, this equals to $3 (.015 x 200 points). Therefore, the cards offer pretty much equal value, but for one you need to spend on travel, and the other you can use to buy whatever you want, without having to worry about not having enough points or when/how to use the points.
Again, the perks that come with some of the cards are what people like most, and the guilt-free feeling of saying that the trip was paid for through points.
Are the best travel credit cards generally associated with a particular hotel chain / airline?
Not necessarily, but the Starwood Preferred Guest card tends to allow most versatility, as points can be transferred to airlines.
It is important to note that cards that say points can be redeemed anytime tend to require more points per flight, or you must book through a specific travel website, where the lowest air prices are not always available.
There is always a catch, but like coupon shoppers, if you research and stay on top of the various specials the different cards offer, you can get some good deals -- e.g., book for half the points.