In my opinion, an investment in some basic temperature control is a better improvement than an investment in a stir plate, bar and erlenmeyer, though the latter is still of value.
If you are pitching really healthy yeast, such as a fresh vial, then the main difference you get with a starter is reduced yeast growth (cells multiply fewer times before entering the anaerobic stage of fermentation), which in turn reduces ester and phenolic production. This can be a good or bad thing, depending on what you're looking for. For example, I recently found Wyeast 3711 (French Saison) to produce wonderful fermentation character with no starter, and it might actually have been reduced for the worst if I had made a starter for that beer. On the other hand, if you're looking for a relatively clean IPA or a lager style beer, those esters will no be appreciated. Similarly, for a high gravity belgian ale, the phenoics might be too strong if you under pitch. As others have mentioned, in high gravity beers, you may also encounter problems with low pitch rates that include stuck fermentation, poor attenuation, and undesirable fermentation by-products.
I also suspect that part of the issue is not how this improves every beer you make where you could tell in a side by side test which had the proper pitch rate; rather, if you don't use a starter, it's more like you increase the chance of a failed batch: with a starter, you eliminate a 1 in 5 or 10 chance of failed or poor fermentation.