People like to stick with what they know, and anyone who used Slide on Reddit will now be that much more inclined to give Lemmy a try because they get an interface that feels familiar. I can see this only benefiting the communities on Lemmy over time as the user base increases. Other popular apps like Boost coming to Lemmy would also draw in new users.

It's great to have a base layer of free, good quality apps to accomplish some goal because it creates a very low barrier to entry. I keep F-Droid installed on my phone because there are times I need a very basic app to do something simple and the risk of malware is inherently lower in an app whose source is public vs private. I can check out the repository and take a look for myself if the permissions it requests are concerning.

That said, there are real advantages to a proprietary app. The developer has a financial incentive to keep the product up to date and add more features to maintain or increase the user base. This benefits not only paid users but also unpaid, ad-supported users.

Like you said, it's about choice. If FOSS is important to you, go ahead and pick one of those clients. If you like snazzy new features or you want to stick with a client you're familiar with, go ahead and do that. Nobody should be shamed or criticized for their choice either way.