I'd argue that "need" is a slightly strong term, but it is worth noting that there are only two volume knobs on the SD-744t which could potentially be limiting. While you don't need to mix between the tracks on the fly, it may be necessary to make adjustments on the recording levels themselves to ensure consistent recording level and quality.
You can always adjust balance later, but what you can't do later is move the noise floor or the max signal level if the audio gets either too quiet or too loud. Inserting a field mixer before the recorder allows for adjustment of recording levels on the fly to ensure that you get maximum quality.
If you are just going to configure it before hand and let it run, a mixer should likely be unnecessary as you won't be making any adjustments anyway, but it can be valuable to have if you either want to be able to monitor and adjust recording levels live during recording or want to be able use more than one audio source per recording channel.