So, I like wearing the suit and tie and you probably think I'm "conformist". I also tend not to judge people based on how they dress or look. I make active efforts to be as non-biased by peoples' physical appearances as possible. Yet I know (from experience) that it's an unreasonable expectation of other people to hold them to this same status and that, in the real world, appearance matters.
What I've learned, the hard way, is that you have to spend a considerable amount of your work time deciding for other people how they perceive you, and this is a subtle art. If you don't manage your image actively, others will manage it for you. This is as true, if not moreso, in a startup than in a big company. At least the social status rules of large companies (as broken and antiquated as they may be) are documented. In a startup, you often have situations where there are 20 employees who all think they're the alpha dog.
Wearing a suit and tie is a very cheap, efficient way to manage other peoples' impressions of you. Sure, it's silly, but the reward-to-cost ratio is very high. It's a lot cheaper and better than, say, sticking around the office till 9:30 when you finished productive work 5 hours ago.