Depends on who you are.
Fan translations very well might be hurting JastUSA’s ability to make money on any individual because they raise the standards, they can’t just keep releasing ZyX sex romps and expect to keep making money. This is likely why they got a deal with Nitro+ (well that, and MangaGamer has got a bunch of kick ass titles compared to Jast release, Shuffle and Higurashi have far more appeal then Family Project and random sex romps). However at the same time, fan translations do a good job at expanding the visual novel community.
For fans of visual novels though, it is absolutely wonderful. Many of the top tier games that we English speakers would have not gotten otherwise we have gotten thanks to fan translators. Say what you will, but I doubt most non Japanese speakers will complain about getting stuff from Type Moon or Key through fan translators then just getting ZyX stuff from Jast. MangaGamer very well might have came into existence due to fan translations creating a market outside of sex romps, and now they are releasing big title after big title.
For the consumer, competition, regardless of the source (fan translations or other companies like MangaGamer), means that the consumer benefits. If Jast suffers because of this competition, even if it is not truly legal, that means that they have to adapt, which can be seen with a deal like their Nitro+ deal. Complaining about “illegal competition” does absolutely nothing because that competition is not going to go away, and honestly, if Jast can’t benefit from the significantly expanding Visual Novel fanbase, then they are doing something wrong.