Moneylender is a lot better than "rarely buy". It's certainly better than Moat, Feast, or Bureaucrat, especially in the absence of other trashing.
The main time I'd get Moneylender is when all the elements are available for a great engine except for a better trasher, like Chapel or Remodel. An example is Village/Smithy + Festival + Workshop + Moneylender. I'd much rather make this engine if Remodel or Chapel is available, but in their absence Moneylender is better than non-engine alternatives, like Smithy-Big Money. This situation is rare, so I rarely get Moneylender. A list of more common types of games with the base set and why I don't get Moneylender is below. In what common situation would you get Moneylender in a base set game to make it more than a "rarely buy"?
Engine -- As mentioned earlier, Moneylender can work as a trasher in absence of Remodel or Chapel, but the chances of a kingdom with a good enough potential engine to beat Smithy-BM with Moneylender as a trasher and no better trasher available is rare.
Workshop/Bureaucrat/Council Gardens Rush-- Moneylender slows down the rush and doesn't have a great benefit since a large number of expensive cards are not required. It also reduces the value of Gardens with fewer cards in the deck.
Witch Game -- Most persons I play against would open Moneylender/Silver if nothing better is available, yet in sims opening Silver/Silver does better. Moneylender can help get Witch earlier, but it interferes with Witch +2 card draws after you get the Witch(es). In contrast, Feast does help here (in absence of anything better), which is the primary reason why I did not list Feast as a "rarely buy".
Smithy/Council/Library: Big Money -- Same issue as Witch with Moneylender interfering. Buying almost anything other than an additional Smithy/Council/Library will hurt them (I realize Library is a bit different and has a few important exceptions).
Militia as Terminal -- This includes Militia alone or with a combination of things like Market, Festival, Lab, and/or Chapel. Opening with Militia/Silver is better, and if you do get multiple actions (for example with Festival), then it's better to use those actions on additional Militia than a Moneylender.
Lab + Terminal -- Moneylender is a viable terminal action here, but there are often better alternatives, like Woodcutter in the less common kingdoms where there is nothing better than lab + (non-militia) terminal. This makes playing Moneylender as a Lab terminal rare for me.
Thief's main utility isn't very big, but in 4 player games it is a good Gardens enabler as it lets you gain up to 3 cards with one play. With later expansions, Thief lets you take key treasures like Platinum and Fools' Gold.
That's a good point with 4-player games that I had not considered. Thief does get more valuable as number of players increases and probably has far more useful situations in 4-player games than 2-player.
Something to keep in mind is that a base only player at 4500 on the ladder is going to be a lot worse than an all cards player on the same spot on the ladder. After all they are only playing other Base only players.
It's been my experience that all other persons I play against who are ranked 4500+ frequently play with more than just base set, as listed in log files. However, I usually leave "allow cards I don't own" unchecked, so when we play it is just base game cards. I'm sure they'd destroy me in a game where most cards are ones I am not familiar with, but I hold my own in base set games, probably winning the majority of games by a small margin. I expect the larger number of cards in additional sets makes it easier to distinguish players at this rank level. This probably partially relates to why there are few base game only players at this ranking. An opponent ranked 1000+ less is likely to be familiar with simple base game strategies, like Smithy-BM or Workshop-Gardens and has a good chance to win in such games, causing a big rankings decrease. However, the 1000+ less player is less likely to be familiar with the best strategy for an all sets game, so the skilled all sets player is less likely to see a rankings decrease.