Despite our best efforts, some cats are pre-disposed to bouts of UTIs. Did your vet mention why your cat was straining? Was it just a case of bacterial infection or is your kitty plugged with struvite crystals? Putting kitty on RC Urinary SO is fine, and that's the maintenance diet that we're going to put my cat Pico on once he's done his 4-6 weeks of Hill's S/D. He was diagnosed with FLUTD, although this encompasses many disorders such as UTI's, idiopathic cystitis etc. ....they all manifest with similar symptoms (i.e., straining to urinate, blood in the urine, spraying for some cats).
If you're unsure of your kitty's diagnosis and the appropriateness of the course of treatment thus far, it's probably advisable to get a second opinion, especially if the treatment has been repetitive as you have suggested. In the meantime, try to feed more canned food. This is inconvenient and pretty expensive but well worth it as it contains substantially more moisture and protein and significantly less magnesium, ash and phosphorus (all dietary factors have been shown to contribute to crystalluria). After having researched a lot of information (published / non-published) online, I have been feeding my boys 1/2 canned and 1/2 dry twice a day, and adding enough warm water to make a gravy or soup. If you're not able to give canned at this point, at least add some water to the dish each meal and get a water fountain. You can even get them at Walmart for about $30 (1/2 price that of big chain stores like Petsmart).
Unfortunately, with FLUTD, the only way to manage it is to give the RX diet (or another high quality food with very low magnesium and ash), increase kitty's water intake a lot (adding it to dry, getting a fountain etc), and to decrease his/her stress levels. There is a lot of supporting literature to support RX diets but this should be done with caution, as they can predispose kitties to calcium oxalate crystalliuria (much more difficult to manage if they get them). I have been on the fence with this topic, but have decided to go with the RC Urinary SO dry in moderation, while adding A LOT of wet food and water mixed in. The Urinary SO contains sodium chloride (salt) to increase the thirst urge in your cat, so you may notice that he/she is drinking and urinating a lot more. Increasing his/her water uptake even more is definitely beneficial. I've been told that putting your non-FLUTD cat on the SO is fine, but I'm very hesistant to do that (re: calcium oxalate crystals), so we will be feeding him Fromm's or one of the products through Natura (haven't decided yet). Regardless, it'll be a low magnesium/ash/phosphorus diet.
Most of this advice is derived from my own research (as a biologist with a MSc), but again, I'm not a DVM so consult with them first.