The heuristic described here is one manifestation of what Polya (1945) and others thereafter refer to as trying a special case. I do not know of a more specific term for the context that you have put forth, but this is often how one approaches a problem if you are initially unsure as to how to solve it in generality.
It is a good way to get an initial foot-hold on a problem, and especially useful in the context of some standardized tests. For example, consider the following:
Question: Player 1 flips a penny $n>0$ times, and Player 2 flips a penny $n+1$ times. What is the probability that Player 2 flips more heads than Player 1?
Since none of the choices depends on $n$, you can consider $n=1$ and see easily that the probability is $1/2$. This allows you to blaze through the question very quickly. (Indeed, this holds for all $n \geq 1$.)
Although testing well is an important life skill, your final question as to whether such problems are advisable is interesting. In my estimation, they can be good in the classroom as a way of scaffolding, as long as you press students (when appropriate) to explain why their reasoning holds for the general case.
As one small example, induction problems are done like this: You show the (hopefully easy) base case, and then complete the proof by setting up and using your inductive hypothesis.
A possible pitfall, though, is not only the absence of reasoning by students (as in the example question above) but also a possible reinforcement around misconceptions of proof by example. (I believe this relates to the recent query about counterexamples in MESE 7466.) For more about some of these misconceptions, check out work by Orit Zaslavsky (google scholar).
Lastly: The issue with guessing answers based on problem statements is pervasive in school mathematics. It ranges from only seeing examples of the form $a + b = \square$ and adopting a misunderstanding of the equal sign as an operator, to problems in which students just try to combine numbers in ways that might be "sensible" (without really engaging in sense-making). Cf. the classic:
Teacher: There are 125 sheep and 5 dogs in a flock. How old is the shepherd?
Student: (thinking: 125+5, 125-5, and 125*5 are too big; it must be 125/5) The shepherd is 25.