I haven't used Pleco that much, so I wonder what you're referring to. Is that:
1. The dictionary app (excluding the dictionary).
2. The built-in (free) dictionaries (PLC and CC-CEDICT).
3. The paid add-on dictionaries.
4. The add-on marketplace.
I think if you want to be fair, you shouldn't compare Pleco to apps based on EDict/JMDict, but rather to apps that can read EPWING dictionaries.
From what I can see and judge:
1. The dictionary functionality in the app doesn't seem any better than EBWin/EBMac/EBPocket [1]. In fact, the EB apps seem to offer more advanced functionality like dictionary groups and different search types (prefix/suffix/infix/exact/full-text).
On the other hand, Pleco is also more user friendly than EBPocket. Replacing dictionary groups by a Chinese-English/English-Chinese is a usability win for 99% of the users.
The Japanese-only documentation and retro homepage probably also deter users from using EBPocket. This is a shame, since the apps are pretty great (they do have English UI). If you just want a dictionary they are miles better than Imiwa/Yomiwa and all the other apps which are based on JMDict.
2. The built-in PLC dictionary seems better than JMDict. It's got better and clearer tagging for the headword and definitions and in-line examples (which you can only get by linking JMDict to another dictionary). These are not issues with JMDict itself though, since it seems to contain all this data (there is even a version that incorporates example sentences from the Tanaka corpus [2]). But most of the JMDict dictionary apps I've seen are not good at displaying it. The EPWING conversions I've found are also not great.
While the PLC dict is great, CC-EDict seems much worse than JMDict.
3. The quality of commercially available Chinese<->English and Japanese<->English dictionaries really depends on the publishers. Compiling and editing good dictionaries take years, and as far as I know Japan has been doing digital dictionaries for a pretty long time. In my eyes, the best general purpose J<->E dictionary (Kenkyusha [2]) is still better than the best commercial C<->E dictionaries, but it is aimed at Japanese speakers studying English and a bit hard to use for beginner Japanese learners.
4. I think the easy to use marketplace is where Pleco really shines. With Japanese dictionaries, you either need to buy the EPWING dictionary in a CDROM (I think it's out of print nowadays) or use the official app, which is pretty limited.
This is the real issue here. I don't know if it's Japanese publishers who don't want to sell the dictionary data to third parties in a marketplace model or the app developers themselves being content with JMDict. It's quite sad, since there used to be a vibrant ecosystem of add-on dictionaries on SD cards for physical electronic dictionaries.
[1]: http://ebstudio.info/manual/EBPocket_iPhone/
[2]: Specifically 研究者新英和大辞典 and 研究者新和英大辞典
1. The dictionary app (excluding the dictionary).
2. The built-in (free) dictionaries (PLC and CC-CEDICT).
3. The paid add-on dictionaries.
4. The add-on marketplace.
I think if you want to be fair, you shouldn't compare Pleco to apps based on EDict/JMDict, but rather to apps that can read EPWING dictionaries.
From what I can see and judge:
1. The dictionary functionality in the app doesn't seem any better than EBWin/EBMac/EBPocket [1]. In fact, the EB apps seem to offer more advanced functionality like dictionary groups and different search types (prefix/suffix/infix/exact/full-text).
On the other hand, Pleco is also more user friendly than EBPocket. Replacing dictionary groups by a Chinese-English/English-Chinese is a usability win for 99% of the users.
The Japanese-only documentation and retro homepage probably also deter users from using EBPocket. This is a shame, since the apps are pretty great (they do have English UI). If you just want a dictionary they are miles better than Imiwa/Yomiwa and all the other apps which are based on JMDict.
2. The built-in PLC dictionary seems better than JMDict. It's got better and clearer tagging for the headword and definitions and in-line examples (which you can only get by linking JMDict to another dictionary). These are not issues with JMDict itself though, since it seems to contain all this data (there is even a version that incorporates example sentences from the Tanaka corpus [2]). But most of the JMDict dictionary apps I've seen are not good at displaying it. The EPWING conversions I've found are also not great.
While the PLC dict is great, CC-EDict seems much worse than JMDict.
3. The quality of commercially available Chinese<->English and Japanese<->English dictionaries really depends on the publishers. Compiling and editing good dictionaries take years, and as far as I know Japan has been doing digital dictionaries for a pretty long time. In my eyes, the best general purpose J<->E dictionary (Kenkyusha [2]) is still better than the best commercial C<->E dictionaries, but it is aimed at Japanese speakers studying English and a bit hard to use for beginner Japanese learners.
4. I think the easy to use marketplace is where Pleco really shines. With Japanese dictionaries, you either need to buy the EPWING dictionary in a CDROM (I think it's out of print nowadays) or use the official app, which is pretty limited.
This is the real issue here. I don't know if it's Japanese publishers who don't want to sell the dictionary data to third parties in a marketplace model or the app developers themselves being content with JMDict. It's quite sad, since there used to be a vibrant ecosystem of add-on dictionaries on SD cards for physical electronic dictionaries.
[1]: http://ebstudio.info/manual/EBPocket_iPhone/ [2]: Specifically 研究者新英和大辞典 and 研究者新和英大辞典