Question: There are linguists that are very much against conlanging, here are two commonly claimed reasons:

1. It's not science.
2. It wastes manpower, time and energy that should have been used to
rescue an already existing rare natural language or do field work.

The latter reason is also common among non-linguists.

What others are there?

Answer: 
Not sure about being “against”, but I’ve seen some linguists dismiss conlang studies due to a strong form of nativism. Suppose you’re convinced that 1) all languages are variations of an universal, hard-wired human capacity; 2) languages can only be acquired “for real” when you’re a child in the critical period; and 3) the one true goal of linguistic science is to understand Language (defined in these terms). If so, then conlangs would be something like fake data. Natlangs are seen as clues to Language, but conlangs could lead you to wrong conclusions.
Creating a language is like writing a novel or creating a sculpture: it can be a fun exercise, and an object of art for people to admire. If you happen to not be into a certain genre, you will probably not be interested in studying it.

I think the same thing applies to constructed languages. I personally do not find them very interesting, with some exceptions, such as Tolkien's languages—but even those I do not find interesting enough to study: I'm just glad they're there, because they make his books and stories all the more immersive and "realistic". (Many other constructed languages, on the other hand, I find crude or unimpressive for various reasons.)

If certain works of art have some sort of physical, historical, or social significance, then that can be another reason to study them. But it can feel like a waste of time if they haven't. I don't like studying a language that I feel I can't learn anything from, as in nothing relevant for how real languages work. This is probably why studying them is normally not a significant part of academic curricula.