Part of improving UX. I personally find that annoying as well. One of the big pain points on this site for me personally, especially for Quantum, which isn't such a popular game, so you do have to send quite a few invites.
Rule of thumb - if I can do something with extra time and clicks (e.g. send 6 requests in 30 seconds to make 6 suggestions instead of sending 1 request in 30 seconds to make 6 suggestions) then the UI is not tailored for that use case and the UX is bad.
Main question would be - why is there a restriction to sending only 1 suggestion every 5 seconds? I assume the main reason is to reduce the amount of requests hitting the server? If so, adding a 'Suggest all' button would actually reduce the number of requests to the server, not increase them. It would mean people would generally get more suggestions to join tables, but I honestly wouldn't care. If anything, I would manage games I want to play more than I do right now. Now it's basically a list of games I know, versus games that I actually want to play at the moment.