I am not a huge fan of Wordpress. It just lacks too many features out-of-the-box that are instead provided via Plugins. It also uses gravatar to handle users profile pictures by default, which I find extremely annoying.
In my opinion, many of the features you get from Plugins, such as a newsmail or photo gallery, should just be integrated in the CMS itself, or guaranteed by Wordpress to work, and without unnecessary clutter in the UI.
Plugins come with a higher chance of security issues, and they are often filled with spam notifications or advertisements to buy a "premium" or "pro" version of the plugin. This is unprofessional and distracting when you are assembling a site for a client.
Another reason for my dislike is, if suddenly a plugin you are relying on breaks, you will have to wait for plugin developers to update it. This is not an ideal situation to place a client in.
Making money on plugin development
While I can understand that developers want to get paid, you can not really expect to make a living from plugin development anyway.
Plugins are usually laser focused on very simple functionality, and these type of plugins take very little time to develop. I would much rather have this functionality integrated in Wordpress itself.
It is also unlikely that a client will want to buy "pro" versions of all the different plugins we are using, plus it is just bad UX to have "go pro" messages cluttering up the controlpanel. Clients are often not very accustomed with web- stuff, and such clutter just distracts them from what they need to focus on.
Making money on plugin development is probably not sustainable in the long run, because there are so many free plugins available. Someone might also get fed up with your spam and just make their own plugin. It is not like Wordpress plugin development is hard.
Luckily, Wordpress uses PHP and MySQL as back-end technologies, so if you have a good newsmail PHP class, it would take little effort make your own plugin to handle sending of newsletters.