Bootlegs can be quite useful; if someone wants a very expensive / unreleased game, then it's imho way better to make it using an already sacced cart than an original. Same for AES: it's better to make conversions with existing loose converts with beat-up labels than with an original SS1 or FFS for example.
@TK86079: The term "bootleg" is mostly used for MVS carts. AES bootlegs exist, but as they need brand new pcbs etc they are more expensive to manufacture. Therefore, people who make them (NCI, DGE, Atomikwave, !Arcade!, ...) also print very high quality artwork. Take a look at a DGE V-Liner, or an Atomikwave Prehistoric Isle II. The paperwork is pretty much the same quality as an original game, maybe even better as printing techniques evolved quite a bit since the 1990s / early 2000s. Furthermore, some even come sealed with reproduction seals. They are called "repros" because most of the games sold as conversions have no manual, a shitty label and a bad quality insert, even though most of these so-called repros aren't even so, but are MVS-only games.
Conversions are games using ORIGINAL PCBs AND chips. As long as there's a PCB or ROM not made by/for SNK inside the cart, it is a boot.
PS: For post-2004 games: NONE of them should be considered a convert NOR a repro. For example, the KBs sold by NGDT or the Xeno Crisis sold by Bitmap Bureau are originals. Not SNK originals obviously as they aren't made by SNK, but originals as they are made by the developers. Even an eprom full cart with a post-it label of Last Hope is original as long as it's made by NGDT. Same applies to SNK protos, btw.
Hope this post helps