A common problem amongst both the PR and non-Japanese Sentai fandoms is that a lot of fans refuse to watch a particular season, if it's missing a Sixth Ranger.
Before the non-Japanese Sentai fandom ever even existed, of course, most of these fans had grown up watching Power Rangers. And since the beginning of Power Rangers, every season had a signature Sixth Ranger. Even the Power Ranger ads on television acknowledged the fact that a sixth ranger was a major staple to the franchise. I even remember seeing a commercial where they were going to air a marathon of all the Sixth Ranger arcs in the Saban Era. The announcer said stuff like "Every season, there's always a sixth ranger. Now you can watch them all on Sunday...." And it was showing off clips of all the Sixth Rangers.
With such a staple engraved into the minds of the fans, the fans ended up expecting every season to have a Sixth Ranger. Fans began to expect each season to run in a particular pattern:
1. Introduction arc
2. Getting to know the rangers. The story doesn't quite start yet.
3. About 12 eps into the series, we get our Sixth Ranger arc! The Sixth Ranger arc is pretty much the first big thing that fans look forward to after seeing the Introduction arc.
4. Now that the Sixth Ranger has joined the team, NOW the story begins and works it's way up to the finale.
Now, let's fast forward to when the non-Japanese Sentai fandom now exists.
I remember when Dino Thunder was first airing, despite it's popularity, many fans were REALLY bummed out when it was revealed that the Triassic Ranger was a power-up for Conner, rather than an actual Sixth Ranger. Some fans even going as far as saying that it was one of the worst things to ever happen in Power Rangers history. Dino Thunder was the very first season, after 12 seasons to not have a Sixth Ranger. Thus, the 11 year old tradition was finally broken, and that didn't sit too well with some fans. Not even additional rangers like Trent made the fans feel better, due to him being #5. Kinda silly if you ask me. =P
Of course, some people didn't count the Spirit Rangers as official rangers, due to them not have any proper human identity, and just manifesting from the three main rangers like a weapon. Thus, some fans considered Jungle Fury to have just five members instead of eight. And like Dino Thunder, it didn't sit well with fans, with some fans calling Jungle Fury to be another major offender to Sixth Rangers like Dino Thunder did (despite airing like 4 years later =P ).
On the Sentai side of things, a huge portion of the franchise has either been bashed or ignored due to the absence of Sixth Rangers. A lot of fans tend to ignore the pre-Zyuranger Sentai, and there are several reasons why. One of the reasons is cause there's no Sixth Ranger. Fans believe that if a Sixth Ranger never arrives, then there will be no story to tell. And that was something that Power Rangers tended to do, only after the Sixth Ranger arrived. Thus, many fans believe that the pre-Zyuranger Sentai are all just lots of nothing. Lots of fans tend to cover this up by saying the shows are "Too old." Which I think is silly. Even additional Warriors like Big One from JAKQ and Black Bison and Green Sai from Liveman don't get acknowledged due to not being #6. Which is why most fans tend to praise Burai from Zyuranger as being the first "true additional warrior in Sentai" due to being #6, and why the DragonRanger/Green Ranger suit is often considered the icon for additional warriors. Also, for fans that say the older shows are "too old," and because they don't have sixth rangers, it's kinda contradictory, if they happened to also think Kakuranger, Abaranger, and Gekiranger were good. =P Though some people consider Gekiranger to have seven members, if Rio and Mele are included. Some people also tend to ignore GoGoFive, due to the lack of the Titanium Ranger, who was quite popular in Lightspeed Rescue, which is rather unfortunate. Since Lightspeed Rescue has such a split fanbase, many fans think that GoGoFive will not be very good without a sixth ranger.
As silly as it sounds, some fans tend to watch only episode 39 of Maskman, just so they can catch the very first Sixth Ranger in history, and just end up ignoring the rest of the season. Lots of fans even go as far as saying that X1 Mask is the only good thing about Maskman.
For a very long time, fans have always wanted to have a female sixth ranger. And many fans were a bit bummed out when both Miu from Go-onger and Gemma from RPM ended up being #7 instead of #6. So apparently, #6 is still considered to be the "better number" amongst fans than #7.
Gosei Knight has been quite a controversial character amongst the fandoms. There are those who say that Goseiger started out bad, but then the show became good after Gosei Knight appeared. But then there are the fans (myself included) who think Goseiger was getting good, but went downhill again due to the arrival of Gosei Knight.
Overall, the Sixth Ranger element has played a huge role on how many fans in the toku fandoms view each series and their stories.