That's a good question. I suppose I am spoiled by my almost always viewing the site on a PC :) There is certainly a balancing act between
- a clean, consistent look for the site
- the need for $\LaTeX$ to express complex mathematics
- giving reasonable loading times for as many users as possible
I would say that, in the example in question, since there was almost no $\LaTeX$, adding a little bit more doesn't do too much harm, and it makes for a consistent look for the site. However, if presented with a page where there were a large number of simple mathematical expressions, all of which could be easily expressible, and more importantly readable, without $\LaTeX$, I absolutely agree that adding it would have a net negative effect, due to the longer loading time.
Now for example, adding $\LaTeX$ to this question, where the OP made an impressive use of HTML to format this mathematics (I believe the reason was that it was posted before the site got $\LaTeX$ abilities), I suppose might be considered a borderline decision. While it was certinaly readable without the $\LaTeX$, I think that the much nicer look and feel of $\LaTeX$ outweighed a shorter loading time.
I also would posit that, as the computing speed of our gadgets improve, the loading time will be less of an issue; and I think that formatting with $\LaTeX$ adds value to the site as a repository for readable questions and answers in the long term.
So, there is not really a right answer; do what you feel is best. I tend to favor using $\LaTeX$ in general, but there are certainly situations where using it might cause more harm than good.